The harms of inequity are rarely overestimated: Comment on Marks and O'Connell (2021)

Marks and O'Connell's (Review of Education, 2021, e3293) claim that an even-handed review of the literature shows that social and economic background does not matter to educational outcomes in advanced economies, relies on questionable scholarship and statistical errors. Much of the evidence against the relevance of social and economic background relies on research that estimates gain scores, where the effects of student background on educational outcomes are much smaller because they are residual effects. They also claim that parental education and occupation are poor measures of student background but do not recognise that this leads to attenuated effects and thus argues against their thesis. Finally, there is a lack of serious engagement with the literature: cherry-picked numbers, summaries of studies that are at odds with the conclusions of the original authors and even two cases where thought experiments are presented as empirical evidence. Research into inequity does at times lack rigour in measurement and analysis, and not accounting for heritable abilities can confound and exaggerate the association between student background or school composition and educational outcomes, but simple calculations suggest that the resulting bias is offset by attenuation due to measurement error.