Fig. 3

a–d Scatterplots of between-country relationships between central tendency (mean achievement and median TICS) and spread (standard deviation of achievement and median absolute deviation of TICS). The white line is the best simple linear fit line, and the gray band is its 95% confidence interval (bootstrapped from 10,000 draws). As there was little between-country variation in median absolute deviation (MAD) of TICS, the x-axis values in b and d were slightly jittered to increase visibility of points. The blue arrow indicates the shift from Norway grade 8 to grade 9