Fig. 9
From: The measure of socio-economic status in PISA: a review and some suggested improvements
Trends in the possession of distinct household possession items, by national income. Lower income countries include countries with a gross national income below USD 28,500 (PPP). This threshold was chosen in order to have an about equal number of countries in the lower- and higher-income group. Only countries/economies that participated in every PISA cycle are included in the analysis. The lower values (compared to a linear trend line) observed in 2003 for many items may be related to the different response format used in that year, which did not distinguish item-non-response from a “no” answer (see notes under Table 1)