 Research
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A quantitative synthesis of the immigrant achievement gap across OECD countries
Largescale Assessments in Education volume 2, Article number: 7 (2014)
Abstract
Background
While existing evidence strongly suggests that immigrant students underperform relative to their native counterparts on measures of mathematics, science, and reading, countrylevel analyses assessing the homogeneity of the immigrant achievement gap across different factors have not been systematically conducted. Beyond finding a statistically significant average achievement gap, existing findings show considerable variation. The goal of this quantitative synthesis was to analyze effect sizes which compared immigrants to natives on international mathematics, reading, and science examinations.
Methods
We used data from the Trends in International Mathematics and Science Study (TIMSS), the Programme for International Student Assessment (PISA), and the Progress in International Reading Literacy Study (PIRLS). We investigated whether the achievement gap is larger in some content areas than others (among mathematics, science, and reading), across the different types of tests (PISA, TIMSS, PIRLS), across academic grades and age, and whether it has changed across time. Standardized mean differences between immigrant and native students were obtained using data from 2000 to 2009 for current Organisation for Economic Cooperation and Development (OECD) countries.
Results
Statistically significant weighted mean effect sizes favored native test takers in mathematics \left({\overline{d}}_{\mathrm{math}}^{*}=0.38\right), reading \left({\overline{d}}_{\mathrm{reading}}^{*}=0.38\right), and science \left({\overline{d}}_{\mathrm{science}}^{*}=0.43\right). Effects of moderators differed across content areas.
Conclusions
Our analyses have the potential to contribute to the literature about how variation in the immigrant achievement gap relates to different nationallevel factors.
Introduction
Immigration has gained increasing attention worldwide in recent years. It has steadily increased in the past five decades, primarily in developed countries (OECD [2010a]). This is especially true for traditional countries of immigration, or those largely defined by a history of settlement through immigration (Buchmann & Parrado [2006]) – the United States, New Zealand, Australia, Canada, and more recently countries such as Germany. In these countries, the stock of the population that is foreignborn has steadily increased since the beginning of the past decade (OECD [2010b]). Immigration is a multifaceted and complex activity. It addresses important demands of the job market, such as filling gaps created by rapidlyaging populations and decreasing fertility rates. Furthermore, it is related to issues of human rights, as immigrants tend to migrate due to reasons of political, racial, economic, or social strife. The flow of people into a given country raises many issues, including the extent to which immigrants become successful members of society. For the youngest of immigrants, success in school is one of the most important indicators of success in society.
The present analysis uses quantitative synthesis methods to examine the extent to which the gap in achievement between immigrants and natives varies at a national level. To our knowledge, only one study has compared the magnitude of the immigrant achievement gap across content areas. Schnepf ([2007]) separately analyzed the three data sets we combined.
Background
One of the most influential factors in the future success of immigrants, particularly children, is education. Internationally, evidence demonstrates that immigrant students are at an educational disadvantage, typically scoring lower on assessments of science, mathematics, and reading, leading to poor educational outcomes such as a low likelihood of participating in preprimary education and low graduation rates (e.g., Ammermuller [2007]; Heus et al. [2009]; Ma [2003]; OECD [2010a]; Portes & MacLeod [1996]; Portes & MacLeod [1999]; Rangvid [2007]; Rangvid [2010]; Zinovyeva et al. [2008]).
Immigrants’ success or failure largely depends on the opportunities they encounter. International educational achievement has been an important factor for economic growth (Hanushek & Kimko [2000]), yet strong evidence suggests educational opportunities are not provided equally to immigrants as they are to natives. Without adequate educational opportunities and, subsequently, adequate pay, immigrants may become a permanent part of the underclass and “foster undesirable subeconomies” to the detriment of society as a whole (Martin [1999], p. 1). Furthermore, the successful integration of immigrants is essential for the maintenance of a stable society, which cannot properly function when large minority groups such as immigrants live in a permanent marginal situation (Christensen [2004]).
Some evidence indicates that immigrant students are more likely than natives of a country to attend lowquality schools (OECD [2010a]). This raises important questions about the quality of education immigrants across the world receive. Due to the increased importance of immigration worldwide, a large number of studies have investigated issues such as employment and earnings outcomes, immigrant adjustment and adaptation, discrimination, and history. Relative to the expansive coverage of the aforementioned subjects, the educational achievement of young immigrants has received less attention in the literature.
In this quantitative synthesis we compute standardized mean differences comparing immigrant students to native students on mathematics, reading, and science in the three major crossnational assessments – TIMSS, PISA, and PIRLS. We use moderator analyses to assess the homogeneity of the gap across OECD countries and how its size relates on various macrolevel dimensions. We examine one general research question, and four specific research questions through these analyses:
On average, is there an immigrant achievement gap?

1.
Does the magnitude of the gap differ across content areas – mathematics, science, and reading?

2.
Does the immigrant achievement gap vary across the three tests – PIRLS, PISA, and TIMSS?

3.
Does the magnitude of the gap differ across grade and age?

4.
Has the size of the immigrant achievement gap changed over time?
Existing research on the immigrant achievement gap
Some research on immigrant education has focused on the immigrant achievement gap and on investigating whether or not this gap exists across various countries. Most analyses compare immigrants’ to native students’ achievement, controlling for a variety of sociodemographic variables such as language spoken in the home, gender, and various proxies for poverty such as books owned in the home and parents’ occupation. To a large extent, an immigrant achievement gap has been found across the board. The literature is inconsistent in its use of covariates. For example, while some studies control for race and ethnicity variables, others do not. It thus becomes challenging to generate theories about the immigrant achievement gap and makes comparisons of the size of the gap difficult. An analysis that investigates this deficit unconditionally avoids treating diverse conditional effects as if they were comparable. In some instances the gap is strongly associated with such variables – so strongly that the gap may become insignificant when these characteristics are entered in statistical models (e.g., Portes & MacLeod [1996]; Warren [1996]). In other studies, these variables do not seem to share variance with the gap (Driessen & Dekkers [1997]), leading authors to conclude that institutional differences such as segregation across schools need to be statistically controlled in order to better understand how immigrant status affects achievement (Buchmann & Parrado [2006]; Christensen [2004]; Dronkers & Levels [2007]; Marks [2005]; Rangvid [2007]; Schnepf [2007]; Wöβmann [2003]). It thus becomes challenging to generate theories about the immigrant achievement gap and strengthens the need for an analysis that investigates this deficit unconditionally.
By and large, research has not indicated whether the immigrant achievement gap is a homogenous phenomenon across countries. Specifically, no systematic effort has yet been made to understand the phenomenon crossnationally, considering possible sources of variation such as content area (i.e., academic subject) and type of content assessed. A systematic analysis is necessary to soundly understand the gap and requires first looking at it unconditionally, as methodologies for controlling demographic variables in published studies vary greatly and make results difficult to compile in a comprehensive manner. Our initial investigations revealed that most publications do not report the size of the unconditional achievement gap (Thompson et al. [2011]), making it difficult to compare findings across studies, and to assess the size of the immigrant achievement gap. Furthermore, an overwhelming number of published articles and reports exploring this phenomenon have used data from the 2000 and 2003 PISA assessments. Other important crossnational assessments, such as TIMSS and PIRLS, seem largely absent. Therefore, while the general consensus is that an immigrant achievement gap exists, the extent to which it varies across populations based on age or the type of content assessed has yet to be examined.
The assessments most often used in the immigranteducation literature differ across several dimensions. The PISA is an assessment of mathematics, science, and reading skills of 15 yearold students while the TIMSS measures both 4^{th} and 8^{th} grade students on concepts of mathematics and science. PIRLS assesses 4^{th} grade students on concepts of reading and literacy. According to the OECD, PISA is not linked to the school curriculum (see [OECD PISA Website]), but rather evaluates “to what extent students at the end of compulsory education, can apply their knowledge to reallife situations and [are] equipped for full participation in society”. The central question here is whether or not students can employ what they have learned in school to situations they are likely to encounter in their daily life – what is the yield of education at or near the end of compulsory schooling? In contrast, PIRLS and TIMSS are tied to curriculum, and evaluate achievement up to a certain point in schooling (see [PIRLS & TIMSS Websites]). Their central aim is to evaluate student knowledge of course content that is actually taught (Hutchison & Schagen [2007]). This diversity in assessment purposes and populations raises the possibility that the immigrant achievement gap reported in existing studies may vary by the age of the students, as well as the purpose of the test or the type of content assessed. As an example, because most studies of 15 yearolds employ the PISA, the average immigrant achievement gap as currently understood in PISA may not extrapolate to younger populations.
Systematic and crossnational approaches to immigrant issues have the potential to highlight different immigrant experiences as well as to reveal international trends. Portes ([1997]) suggests such research is useful for three specific reasons:
…first, to examine the extent to which theoretical propositions “travel,” that is, are applicable in national contexts different from that which produced them; second, to generate typologies of interaction effects specifying the variable influence of causal factors across different national contexts; third, to themselves produce concepts and propositions of broader scope. (p. 820)
Our study targets Portes’s third point. We believe this study will contribute to the growing literature on the immigrant achievement gap.
We do not include other macrolevel factors as moderators, as the information available in most administrations of the three tests is limited, namely, information on the origin country of immigrant pupils is not available. Recent literature indicates that in order to fully understand immigrants’ outcomes in a destination country, we must account for both origin and destination effects, the socalled ‘double perspective’ approach (Dronkers et al. [2014]; Levels & Dronkers [2008]; Levels et al. [2008]; see Limitations and Conclusion).
Methods
Our quantitative synthesis did not retrieve data from publications of secondary analyses, which is the traditional mode of data retrieval for quantitative syntheses, or metaanalyses. Rather we computed mean differences directly from raw data. The primary reason for this was that, in the main, existing studies of the immigrant achievement gap did not provide the information necessary for us to compute effect sizes, which would have limited our analysis significantly. In addition, because the international databases are available free of charge from the IEA (for [PIRLS] & [TIMSS]) and OECD websites (for PISA), secondary analyses are not necessary. Using the primary datasets reduces potential sources of errors introduced when compiling data from published studies that may have used different methods of data extraction and aggregation. Still, although data retrieval for this study was less conventional, other methods employed for the study were the same as those used in typical metaanalyses. Within content areas (i.e., mathematics, reading, and science), we obtained single effect sizes for each grade within each OECD country.
We included only OECD countries in this analysis for several reasons. First, at least a third of all immigrants across the world move from developing to developed countries or from one developed country to another (UNDP [2009]). The OECD is an organization composed of some of the world’s most advanced and developed countries, many of which experience significant immigration. Second, in the past decade, the OECD has devoted significant attention to the issue of immigration within its member countries. It has released yearly publications such as the International Migration Outlook, Where Immigrant Students Succeed – A Comparative Review of Performance and Engagement in PISA 2003, and Equal OpportunitiesThe Labour Market Integration of the Children of Immigrants (see for example OECD [2006a], [b], [c], [2010a], [b]). We focused on the immigrant achievement gap as a phenomenon particular to a specific type of immigration – country to country as opposed to within country migration^{a}. The latter has not been well investigated in the immigrant achievement gap literature and cannot be studied given the data available from the three testing programs.
Effect sizes were computed with various software using data available in the original datasets for PIRLS, PISA, and TIMSS. An immigrant was defined as a student not born in the country of testing. Immigrant status is derived from a “yes” or “no” question included in all three assessments that asks the student whether or not they were born in the country of testing.
First, we computed means and standard deviations for native and immigrant students for each country using the International Data Base (IDB) Data Analyzer (IDB Analyzer (Version 2) [2009]), an application developed by the IEA Data Processing and Research Center to be used in conjunction with SPSS. The IDB Analyzer uses the total student weight and five plausible values for each outcome (OECD [2006b]; OECD [2003a]; Martin & Kelly [1996]; Martin et al. [2003]) to obtain population estimates of mean performance as well as an estimate of the variance of this quantity at the country level. We then computed mean differences, effect sizes, and effectsize variances using Excel because the IDB analyzer does not compute effect sizes or effectsize variances. All other calculations and analyses were conducted in R (R Development Core Team [2011]; R Core Team [2014]) using the metafor package (Viechtbauer [2010a], [b]).
Moderators
Moderators are variables that may affect or relate to the sizes of effects. The three moderators in this study (year, test, and, grade) were selected according to gaps in current research. First, as previously discussed, current knowledge based on studies of the immigrant achievement gap may only be generalizable to 15year olds tested on concepts attached to realworld applications. Thus, we examined the test – PISA, TIMSS, or PIRLS –and related features – year implemented and grade assessed – as moderators. We investigated whether the immigrant achievement gap has changed across time in the past decade, considering that most existing studies have only employed the 2000 and 2003 PISA data. In regression analyses moderators test and grade were treated as discrete variables^{b} and year was continuous and centered at 2000 (the first year of data).
Effect sizes
To quantify the immigrant achievement gap, we calculated mean differences at the country level between immigrant and native test takers. Considering differences among measures and scales for outcomes, a standardized mean difference was the most reasonable choice for computing effect sizes given the aggregated format of the data. The unbiased standardizedmeandifference effect size is
where {\overline{Y}}_{i}^{\mathrm{N}} and {\overline{Y}}_{i}^{\mathrm{I}} are the sample mean outcomes for the respective native and immigrant samples of the i^{th} test administration, {n}_{i}^{\mathrm{N}} and {n}_{i}^{\mathrm{I}} are the respective native and immigrant sample sizes from the i^{th} test administration, and S_{ i } is the pooled standard deviation of the i^{th} sample (Hedges [1981])^{c}. Therefore, a positive effect size is interpreted as an achievement gap which favors native test takers, and a negative effect size favors immigrant examinees. As shown in Hedges ([1981]) and Borenstein ([2009], p. 226), the variance of d_{ i } is
Data were gathered for all TIMSS, PISA, and PIRLS administrations during the years 2000 to 2009 for all countries that were members of the OECD as of 2011. This resulted in an initial set of 542 unique effect sizes classified by country, year, test, grade, and content area. Except for when comparing across content areas, samples for which effect sizes were computed are independent. Typically, during any given test administration, students complete tests in multiple content areas at one time. This results in a dependency in responses across content areas, which further translates to dependence among effect sizes. To address this issue, we analyzed the three content areas separately.
Some test administrations had very low immigrant sample sizes, the lowest of which was only two students. Because the consistency and efficiency properties of the standardized mean difference rely on large sample statistical theory, we excluded samples which had an immigrant sample size (n^{I}) less than 30. As a result, 29 effect sizes were excluded (roughly 5% of the original sample), bringing the number of effect sizes used in the quantitative synthesis to 513.
Analyses
A typical method of choosing a quantitative synthesis model (fixed or random effects) is to determine the extent of homogeneity among effect sizes. Multiple methods have been proposed, the most common being the homogeneity test referred to as the Q test. The formula for Q, as shown in Shadish and Haddock ([2009]), is
If all studies are homogeneous and share a common effect size, Q will be approximately distributed as a chisquare distribution with k − 1 degrees of freedom (df) (Hedges [1992]). The null hypothesis tested by the Q statistic is that all effect sizes are homogenous and any variability results from sampling error. Large values of Q suggest that our collection of effect sizes is heterogeneous. Three Q statistics – one for each content area – are presented in Table 1.
A secondary index for analyzing effectsize homogeneity is the I^{2} index, which “…describes the percentage of total variation across studies that is due to heterogeneity rather than chance” (Higgins et al. [2003], p. 558). We calculate I^{2} as
Higgins et al. ([2003]) interpret I^{2} values as showing no variation, low variation, moderate variation, and high variation for cutoffs of 0%, 25%, 50%, and 75%, respectively. As with results from the Q test, resulting I^{2} values (see Table 1) indicate that randomeffects estimation would be appropriate.
The randomeffects estimate of the mean can be interpreted as an average effect size because it does not assume the population of effect sizes can be completely explained by a unique effectsize representation. Among many other sources, Hedges and Vevea ([1998], p. 493) present a general formula for calculating the randomeffects mean effect size as
where w_{ i }^{*} is the randomeffects weight and is calculated as {\left({v}_{i}+{\widehat{\tau}}^{2}\right)}^{1}. The v_{ i } term is given in (2). The addition of {\widehat{\tau}}^{2}, typically referred to as the betweenstudies variance, represents the presence of true variability among studies beyond sampling error. In place of the term ‘betweenstudies variability’ commonly used in metaanalysis applications, we will refer to betweeneffects variability. The betweeneffects variance component must be estimated; we used the commonly implemented DerSimonian and Laird ([1986]) estimator
Last, the conditional variance of the randomeffects mean is
Using (5) and (7), a 95% confidence interval about the randomeffects mean can be formed as
and a 95% prediction interval can be calculated as
In an effort to explain betweeneffects variability, we examined mixedeffects regression models. These models incorporate regression coefficients that associate study characteristics (i.e., moderators) to study outcomes while allowing for unexplained variance in the model (Raudenbush [2009]). Our mixedeffects regression models consider effect sizes as outcomes, and study characteristics (such as test) as moderators of the variability among effect sizes. For each we provide a Qmodel statistic, denoted as Q_{M}(df). This statistic assesses the amount of total variation explained by the model. When effect sizes are wellexplained by the moderators, Q_{M} will be large. We also provide a Qerror statistic, denoted as Q_{E}(df). This statistic assesses the amount of total variation not explained by the predictions when a fixed effects model (with explained variation not incorporated) is examined; lower Q_{E} values are desired. Result for Q_{M} and Q_{E} can be found in Tables 2, 3 and 4.
Results
Overall analyses
Figure 1 provides error bar plots for all effects by content area and shows that the ranges of effects within content areas were fairly similar. The lowest effects were medium in magnitude and negative, representing cases where immigrants outperformed natives, while the highest effects were large and positive. Across all content areas, over 80% of effect sizes were positive, indicating an achievement gap which favored native test takers. Furthermore, across all content areas no large negative effects were seen. Last, over 75% of the effects were statistically significant at the α = .05 level.
Table 1 provides effectsize homogeneity information for each content area. All three data sets had statistically significant Q statistics. In addition, the average I^{2} across the content area was 97%, which is very large. Both homogeneity indices agree that effect sizes for all three outcomes are heterogeneous. As previously stated these indices also indicate the appropriateness of adopting a randomeffects model.
Table 5 provides the betweeneffects variances, as well as randomeffects means and their associated 95% confidence and prediction intervals for all content areas. Mean effect sizes for mathematics and reading data were identical (both equal to 0.38). This indicates a smalltomoderate overall effect favoring native students. The mean effect for the science data was slightly larger (0.43) and also favored native students. All means were statistically different from zero. Last, the betweeneffects variances \left({\widehat{\tau}}^{2}\right) were fairly large and similar across all subjects.
Mixedeffects regression models
As part of the mixedmodel analyses, all data sets were checked for multicollinearity among moderators. Bivariate correlations were calculated for all predictors (see Table 6). In all three content areas the largest correlation, by far, was between grade and test. This occurs because PISA is given only to 15 year olds, PIRLS to 4th graders, and TIMSS at multiple grade levels. For this reason, we did not include grade and test simultaneously as moderators in the models. Beyond this high degree of multicollinearity, other moderators had low degrees of dependence as determined by moderatelylow bivariate correlations and low variance inflation factors (all were less than two).
Tables 2, 3 and 4 provide regression coefficients, standard errors, and probability values for both models (excluding either grade or test) in each of the three content areas.
Mathematics data
Results for mathematics data differed based on whether grade or test was used as a moderator in the model. When test and year were modeled (i.e., excluding grade), the only statistically significant moderator of the size of the immigrant gap was test\left({\widehat{\beta}}_{\mathrm{test}}=0.152\right). This implies that, holding year constant^{d}, the average difference among effect sizes between the PISA and TIMSS data was 0.152. Specifically, the immigrant achievement gap is 0.152 standard deviations larger for TIMSS data than for PISA data.
When grade and year were included as moderators (i.e., excluding test), results were similar for the slopes representing the moderator grade ({\widehat{\beta}}_{\mathrm{grade}\phantom{\rule{0.25em}{0ex}}\left[1\right]}=0.237 and {\widehat{\beta}}_{\mathrm{grade}\phantom{\rule{0.25em}{0ex}}\left[2\right]}=0.274). The predicted size of the gap for 4^{th} graders was 0.64 standard deviations, controlling for year. The immigrant achievement gap in math was 0.237 standard deviations smaller for 8^{th} grade test takers than for 4^{th} grade test takers, and 0.274 standard deviations smaller for 15 year olds than for 4^{th} graders. Taking the difference between these slopes gives the difference between gaps for 8^{th} graders and 15 year olds, which is a negligible 0.001 standard deviations. These values show that the size of the immigrant achievement gap is lower for all older examinees, by about onefourth of a standard deviation.
Both models explained a significant amount of heterogeneity in the math gaps, as indicated by Q_{M}(3) = 16.3, p <.05 and Q_{M}(2) = 8.0, p <.05, respectively. However, both Qerror statistics (Q_{E} from the fixed model) were quite large and statistically significant (see Table 2), indicating much effectsize variability has yet to be explained.
Reading data
In contrast to the results for mathematics, results for the reading data did not significantly differ based on whether grade or test were entered in the model. In both instances year was a significant moderator \left({\widehat{\beta}}_{\mathrm{year}}=0.017\right). On average, the immigrant achievement gap has decreased by 0.017 standard deviations each year since 2000. This result is best interpreted as a weak, general trend over time rather than a yeartoyear difference because none of the examinations studied is offered every year. We examine this result more closely with a cumulative quantitative synthesis in Appendix B. The reading model explained a significant amount of effectsize heterogeneity even given a large degree of uncertainty (Q_{M}(2) = 7.8, p <.05). This Q_{M} result was the same for both grade and test models. As with the mathematics models, the Qerror statistic (Q_{E}) was quite large and statistically significant (see Table 3), which means a large degree of effectsize variability was not explained by the predictors.
Science data
For science, the significance of the moderators in both models (i.e., with grade or test) was similar. Both test and grade explained a significant amount of effectsize heterogeneity. The slope for test\left({\widehat{\beta}}_{\mathrm{test}}=0.183\right) reveals that the average effect size was 0.183 larger for TIMSS than PISA. In the case of grade ({\widehat{\beta}}_{\mathrm{grade}\phantom{\rule{0.25em}{0ex}}\left[1\right]}=0.166 and {\widehat{\beta}}_{\mathrm{grade}\phantom{\rule{0.25em}{0ex}}\left[2\right]}=0.268) results were similar to those for the mathematics data. The immigrant achievement gap was 0.166 standard deviations larger for 4^{th} grade test takers than for 8^{th} grade test takers, and 0.268 standard deviations larger for 4^{th} graders than for 15 year olds. Taking the difference between these slopes gives the difference between gaps for 8^{th} graders and 15 year olds, which is about 0.10 standard deviations. Given the intercept of 0.65, these results suggest that the immigrant achievement gap is greatest in grade 4, is about 25 percent lower for 8^{th} graders and another 16 percent lower for the 15year olds. Both science models explained a significant amount of effectsize heterogeneity, as respectively indicated by Q_{M}(3) = 18.9, p <.05 and Q_{M}(2) = 13.8, p <.05. As with the mathematics and reading models, both Qerror statistics (Q_{E}) were quite large and statistically significant (see Table 4).
Model fit
We also tested a series of assumptions for each linear model. First, many potential influential points had been eliminated by virtue of their small sample size of immigrant students (5% of the total set of effects were excluded, as previously mentioned). Many of the excluded effects were large. Leverage plots were also examined to determine if any influential points existed. Within the remaining effect sizes, several potential influential points were located, but their influence was minimal based on information derived from the leverage plots. Ultimately we did not exclude any additional observations as these potentiallyinfluential points are likely not products of measurement imprecision (see our previous discussion on excluding data from small samples).
Normal quantilequantile plots confirmed approximate normality of residuals for all content areas, and partial residual plots confirmed approximate linearity of continuous predictors related to effects, in all content areas. All preliminary assumption checks were completed using the car package in R (Fox & Weisberg [2011]). As in all modeling scenarios, model fit can always be improved. First, though all models explained a significant amount of variability in effects (as shown by Q_{M}), all model fit tests (Q_{E} results) were very large and statistically significant. This excessive unexplained variability may be explainable if we were to test other moderators (see Limitations). Second, varianceexplained values for all six models, denoted as {R}_{\mathrm{meta}}^{2}, were all small, ranging from almost zero to .08 (values are not shown here), further indicating the potential for other moderators to explain effectsize variability. This measure compares the variability explained by the model with no moderators to the variability explained by a model with moderators (see Aloe et al. ([2010]) for more information). Both of these indicators of model fit suggest further variation remains in all three sets of contentarea effect sizes.
Discussion
We found significant overall mean effect sizes for mathematics \left({\overline{d}}_{\mathrm{math}}^{*}=0.38\right), reading \left({\overline{d}}_{\mathrm{reading}}^{*}=0.38\right), and science \left({\overline{\mathit{d}}}_{\mathrm{science}}^{*}=0.43\right), all of which are moderate in magnitude. Prediction intervals suggested that the bulk of the effects in all areas are likely positive, favoring native students. Only 8 percent (for science) to 13 percent (for mathematics) of effects are likely to be below zero. This addresses the overarching research question and indicates that, in fact, an immigrant achievement gap exists for all assessed content areas in favor of native students. The gap for science is slightly larger than the mathematics and reading gaps, which are empirically identical. While a difference across content areas has never been previously tested with metaanalytic methods, other authors have posited such a pattern. For example, Schnepf ([2007]) argued that the gap would likely be larger for reading than mathematics because assessments of mathematics require fewer linguistic skills than reading assessments; this would relate directly to immigrant students’ proficiency in the language of testing.
This quantitative synthesis does not completely support this notion; rather, it suggests that immigrant students are at an equal disadvantage in reading and in mathematics when compared to native students. Yet the logic presented by Schnepf ([2007]) may explain the significantly higher gap in science. Perhaps the language used in mathematics is more universally understood, while context in both math and reading assessments may aid immigrant test takers who are nonnative language speakers in deriving meaning in order to successfully respond to questions. Further, the content in a mathematics assessment is often numerical, and to the extent that the immigrant students’ native countries use the same number system as the country of testing, this type of assessment may be less daunting than a science assessment. Unlike mathematics items, science items may tend to be word problems that include technical language in the language of testing. They also may not provide as much context as a reading passage. For example, immigrant students who do not speak the language of testing well may be able to create meaning from the reading passage. In other words, not knowing the meaning of some words may not be as detrimental when the item is longer and has context as opposed to when the item is short and lacks context or includes technical terms (which may be likelier for science items). However, such an explanation only applies to nonnative language speakers. Some immigrants do speak the language of the test as a first or additional language. Perhaps this finding also hints at potential differences in quality of science curriculum and instruction between origin and destination countries. If immigrant students have been exposed to a poorer quality science instruction in their native countries, for example, then this may exhibit itself in a science immigrant achievement gap on assessments given in the destination country.
Six separate regression models, two for each content area, addressed our subsequent research questions. While statistical significance of the moderators varied, some similarities were found across the models, specifically between mathematics and science. The achievement gap was larger in TIMSS than PISA for both mathematics and science by about one to two tenths of a standard deviation. The gap was also smaller for older immigrant children by about two tenths of a standard deviation in both math and science.
Next, only one moderator was significant for the reading effects – year of testing. Although this quantitative synthesis is in the main crosssectional, the significance of this moderator would indicate a possible weak trend in which the gap in reading has decreased from the beginning to the end of the last decade (see Appendix B for a slightly different perspective on the matter).
Because few studies have examined macrolevel differences in the immigrant achievement gap, it is difficult to make strong theoretical interpretations of the findings. Perhaps the most significant findings are the differences across grades and tests. The fact that younger students show a larger immigrant achievement gap is not necessarily intuitive, since it is commonly believed that young children adapt to new environments more easily and learn new languages more quickly than older students. The difference we found may reflect the composition of student populations in later grades, which include those who have not dropped out of school or who have the means and the support at home to stay in school, and are thus possibly the most advantaged in a given country. This may imply that academic differences between native and immigrant students at the highest levels of privilege are still present but narrower, although our data are not disaggregated to a level where analysis of this hypothesis is possible.
The difference in the gap magnitude between TIMSS and PISA may be in part due to the type of content assessed. Specifically, TIMSS assesses the effectiveness of the curriculum whereas PISA evaluates the extent to which pupils at the end of compulsory schooling can apply what they have learned to situations they will likely encounter in their daily lives. Content assessed in TIMSS evaluates formal mathematics knowledge, whereas items in PISA are more applied in nature as they pose realworld scenarios that require mathematics. Perhaps immigrant students fare better on items that tell a story, provide more context, and allow them to apply their experience and knowledge, such as those in PISA. Coupled with the finding that older immigrant children exhibit a narrower gap, this may indicate that immigrant adolescents who have not yet dropped out of school are nearly as ready for the workforce (as measured by PISA) as native students. Our findings seem to suggest larger disparities between younger and older students when assessed with TIMSS than PISA.
Limitations
Several considerations suggest the use of caution when making inferences from this analysis. First, we were limited because most administrations of the three assessments did not collect country of origin information from immigrant pupils. For this reason, we could not investigate macrolevel characteristics of the countries in this study. The most recent research in this area indicates that both origin and destination macrolevel variables must be investigated to fully understand the immigrant achievement gap (Levels & Dronkers [2008]; Levels et al. [2008]; Dronkers et al. [2014]). Second, the generalizability of this study is limited to OECD countries, although our initial investigations also found an overall significant mean immigrant achievement gap with a wider set of countries (Thompson et al. [2011]). Third, because we defined immigrants as students not born in the country of testing, we are studying by definition only firstgeneration immigrants (Rumbaut [2004]). Fourth, in the three testing programs, countries are permitted to exclude students who are nonnative speakers of the testing language and who have received less than one year of instruction in that language. This study, as any other employing data from the PIRLS, PISA, and TIMSS, is representative of students who have a certain degree of proficiency in the language of testing. Fifth, some of the variation in effects found across test content may be due to the differing methodologies employed in PISA and TIMSS for calculating variance rather than an observed effect in the population. Finally, our quantitative synthesis examined the extent to which the immigrant achievement gap varied by subject. To address such a question, we compared reading, science, and mathematics scores that are not on the same scale, although standardized effect sizes in part address this issue.
We have suggested reasons for possible gap differences using several moderators. Although characteristics of an immigrant student, such as their nonnative language speaker status, may contribute to the existence of a gap, they are most certainly not the only source, as previously discussed. Strong evidence has shown inequities in the quality of the education that immigrants are provided in destination countries (e.g., Conchas [2001]; Crul & Holdaway [2009]; Lee [2002]; Minoiu & Entorf [2005]; OECD [2010a]; Schneeweis [2006]). Although immigrant students may be at an academic disadvantage due to their individual characteristics, such as socioeconomic status and native language, the experiences they have had in both their origin and destination countries have an effect on the immigrant achievement gap. Finally, as we did not analyze studentlevel data, we did not investigate any student or school correlates of the immigrant achievement gap. Thus, it is difficult to conclusively discuss all possible sources of the gap. In the future, malleable factors must be investigated in order to better understand how to close the gap. More than likely, factors found at the school level will have the most potential for reducing or eradicating this deficit.
Conclusions
One of the aims of this quantitative synthesis was to examine the extent of the homogeneity of the immigrant achievement gap from a macrolevel perspective. We found that the immigrant achievement gap is a very heterogeneous phenomenon and varies by grade and type of content assessed. It also varies by year (for reading). Thus even though gaps are present on average, they are not constant across all conditions and groups of students. In a small percent of populations, the gaps favor immigrants. Intuitively, the size of the science gap in comparison to the reading and mathematics gaps may make sense. Science assessments may include more complex and technical language than mathematics and reading assessments. Future research should investigate the content of the assessments as well as include itemlevel analyses in order to better understand what features of mathematics and reading assessments yield a smaller immigrant achievement gap than science assessments. The same applies to the type of content assessed in PISA and TIMSS, as evidence presented here suggests immigrants perform less poorly on PISA than TIMSS (relative to natives).
Most analyses to date have questioned whether or not a gap exists across countries, often controlling for studentlevel variables such as race, ethnicity, level of poverty, and native language. Our analysis demonstrates that, on average, there is a gap for the three core content areas across countries. Importantly, singlelevel analyses that control for studentlevel variables cannot answer all questions about what may explain the immigrant achievement gap. Because the gap is not a studentlevel phenomenon, in that no individual student him or herself can exhibit a gap, future questions about the sources of this deficit must analyze the gap as a schoollevel phenomenon. Further, Dronkers et al. ([2014]) emphasize that “contextual features of both origin and destination countries do affect the educational performance of migrant children, and must be part of any explanation of migrant children’s school success.” (p. 2). Immigrants do not arrive in destination countries as a blank slate. Factors such as their educational experiences and reasons for migration influence their degree of success in the destination country. Characteristics of the origin country such as political stability, level of economic development, and length of compulsory education have shown significant effects on the educational achievement of immigrants in the destination country (Levels & Dronkers [2008]; Levels et al. [2008]; Dronkers et al. [2014]). To this end, future studies should continue to investigate possible moderators of the immigrant achievement gap at a national level from both origin and destination countries.
This article provides the most systematic investigation of the immigrant achievement gap to date based on three critical databases. Our analyses investigate correlates of the gap at a macro level. Our findings are consistent with the existing literature which has continuously reported an immigrant achievement gap. Our findings may allow researchers to now focus on investigating malleable factors to address this academic deficit between immigrant and native students instead of continuing to focus on whether or not a gap exists between these students. We hope that our results provide aid organizations with evidence on what variables are associated with the gap so they can tailor interventions to ameliorate the immigrant achievement gap at a national level. Future research should begin to identify further malleable factors at the school and country levels in order to address the academic deficit between immigrant and native students.
Endnotes
^{a}According to the United Nations Development Programme, almost four times as many people move within countries as across countries (UNDP [2009]).
^{b}For test, PISA was coded as “1” and TIMSS and PIRLS were coded as “0.” A third code was not necessary because TIMSS and PIRLS data were never analyzed together because different participants are tested in the two programs. For grade, we created dummy variables for 4^{th} graders (reference group), 8^{th} graders, and 15year olds.
^{c}The standard deviation is {S}_{i}=\sqrt{\frac{\left({n}_{i}^{\mathrm{N}}1\right){\left({S}_{i}^{\mathrm{N}}\right)}^{2}+\left({n}_{i}^{\mathrm{I}}1\right){\left({S}_{i}^{\mathrm{I}}\right)}^{2}}{{n}_{i}^{\mathrm{N}}+{n}_{i}^{\mathrm{I}}2}\phantom{\rule{0.25em}{0ex}}}, where {S}_{i}^{\mathrm{N}} and {S}_{i}^{\mathrm{I}} are the respective standard deviations of the native and immigrant samples for the i^{th} sample.
^{d}Henceforth we will not repeat the phrase “holding all other moderators constant” for the sake of brevity.
Appendix A List of OECD countries in quantitative synthesis

1.
Australia

2.
Austria

3.
Belgium

4.
Canada

5.
Chile

6.
Czech Republic

7.
Denmark

8.
Estonia

9.
Finland

10.
France

11.
Germany

12.
Greece

13.
Hungary

14.
Iceland

15.
Ireland

16.
Israel

17.
Italy

18.
Japan

19.
Korea

20.
Luxembourg

21.
Mexico

22.
Netherlands

23.
New Zealand

24.
Norway

25.
Poland

26.
Portugal

27.
Slovak Republic

28.
Slovenia

29.
Spain

30.
Sweden

31.
Switzerland

32.
Turkey

33.
United Kingdom

34.
United States
Appendix B Cumulative metaanalyses
While investigating year as a predictor, we became interested in how mean effects varied over time for each content area. Therefore we completed cumulative metaanalyses for each subject. Cumulative metaanalyses include multiple, successive metaanalyses for each time point (in our case, year) of data. For example, our data begins at year 2000. At the first time point, only effects based on tests given in 2000 were metaanalyzed using the randomeffects procedures described above. Next, the following time point (i.e., year = 2001) is considered and the same process is completed using effects from 2000 and 2001. This process is then repeated for all time points through 2009. The main advantage of performing a cumulative metaanalysis is the ability to see the stabilization (or lack thereof) of mean effects over time (here, across years of testing).
Figure 2 provides the cumulative metaanalyses for all content areas. As time progresses, confidence intervals typically decrease in size, implying a more precise mean estimate. This is expected as, over time, the number of effects used to calculate the mean increases. However, in a few instances, going from one year to the next, k did not change because the given subject was not tested between those years but other(s) were. These duplicate points were nonetheless included to ensure comparability across the three plots.
Overall, results for all content areas showed fairly stable mean effects, suggesting the gap has been fairly consistent over the period from 2000 to 2009. This is confirmed by the overlap of the confidence intervals across all years, for each subject. One exception may be for the reading data, where a practically significant jump (i.e., an increase in the gap) of about onetenth of a standard deviation was seen from 2000 to 2001. This reflects the weak, but statistically significant effect of the year moderator for the reading model. Practically speaking, this may mean that although the gap in reading increased between the years 20002001, it stabilized over time. This initial jump followed by subsequent decreases may have manifested itself in a negative effect in the reading model when, in fact, the gap was consistent across the last decade for reading as it was for science and mathematics. From a policy standpoint, this suggests that efforts to address the deficit between immigrant and native students in the core subjects have not closed the achievement gap in the past decade.
Abbreviations
 PISA:

Programme for International Student Assessment
 TIMSS:

Trends in International Mathematics and Science Study
 PIRLS:

Progress in International Reading Literacy Study
 OECD:

Organisation for Economic Cooperation and Development
 UNDP:

United Nations Development Programme
 IDB:

International Data Base
 CIA:

Central Intelligence Agency
 df:

Degrees of freedom
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Andon, A., Thompson, C.G. & Becker, B.J. A quantitative synthesis of the immigrant achievement gap across OECD countries. Largescale Assess Educ 2, 7 (2014). https://doi.org/10.1186/s4053601400072
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DOI: https://doi.org/10.1186/s4053601400072