The UK Times today has a report on new research into trends in social mobility and the effect of education and social class. The research finds that social mobility declined between 1958 and 1970, and has not improved since then (boo!). But the Times focuses on a peripheral part of the research, which looks at a recent cohort of young children tested on a cognitive ability scale (actually vocabulary) at age 3 and 5. According to the Times article:
The authors said: “Those from the poorest fifth of households, but in the brightest group at age 3, drop from the 88th percentile on cognitive tests at 3 to the 65th percentile at age 5. Those from the richest households who are least able at age 3 move up from the 15th percentile to the 45th percentile by age 5. If this trend continued these children from affluent backgrounds would be likely to overtake the poorer, but initially bright, children in test scores by age 7.”
The article also contains a graph to illustrate this point, which unfortunately is not included in the online version. However, it is evidently based on Figure 4 in the original research report, available as a pdf file here.
Now, take a look at Figure 4, and don’t all shout at once: regression towards the mean! The graph is almost a textbook illustration of what we would expect to find if we took test data from the extremes of two groups with different mean performance and then retested them at a later date.
This doesn’t prove that this is simply a case of regression, but it is an obvious possibility which needs to be examined. As far as I can see (correct me if I am wrong) the authors do not consider it, but it is hard to tell, as the report is written in statisticese, a language only distantly related to English.