Developmental changes in cognitive ability: Implications for value added measures

2 Jan

A paper published in the most recent issue of the journal Intelligence has important implications for value added measures of teaching. Here is the abstract, I have underlined the relevant sentences:

“Low socioeconomic status (SES) children perform on average worse on intelligence tests than children from higher SES backgrounds, but the developmental relationship between intelligence and SES has not been adequately investigated. Here, we use latent growth curve (LGC) models to assess associations between SES and individual differences in the intelligence starting point (intercept) and in the rate and direction of change in scores (slope and quadratic term) from infancy through adolescence in 14,853 children from the Twins Early Development Study (TEDS), assessed 9 times on IQ between the ages of 2 and 16 years. SES was significantly associated with intelligence growth factors: higher SES was related both to a higher starting point in infancy and to greater gains in intelligence over time. Specifically, children from low SES families scored on average 6 IQ points lower at age 2 than children from high SES backgrounds; by age 16, this difference had almost tripled. Although these key results did not vary across girls and boys, we observed gender differences in the development of intelligence in early childhood. Overall, SES was shown to be associated with individual differences in intercepts as well as slopes of intelligence. However, this finding does not warrant causal interpretations of the relationship between SES and the development of intelligence.”

It is well understood that children in a classroom start at different levels, valued added assessment attempts to control for this by comparing gain scores. In other words, the child’s test score at the beginning of the school year is subtracted from the child’s score at the end of year. The increase is assumed to be the value added to the student by the teacher.

However, this stands on the assumption that children learn at the same rate. This paper (“Socioeconomic status and the growth of intelligence from infancy through adolescence”) tells us that the slope of of the line between beginning and end of year test scores is related to social class. For our purposes here we need not concern ourselves with the direction of causality or why this correlation exists. All we need to know is that scores on intelligence tests are strong predictors of academic achievement. Thus, we can predict that, in general, students from higher socioeconomic backgrounds will show greater value added and these measures will be unfair to teachers who teach children who live in poverty. Overtime, this will create a disincentive for our best teachers to work with the children who most need their help.


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