Dennis et al. (2009). Why IQ is not a covariate in cognitive studies of neurodevelopmental disorders.” – Article summary

Neurodevelopmental disorders are different from adult acquired disorders involving traumatic brain injury because they involve no period of normal development. Any IQ score in a neurodevelopmental disorder is confounded with the condition. The IQ score can never be separated from the effects of the condition.

Early on in intelligence testing, IQ was seen as a latent variable. Intelligence, as measured by ‘g’ was believed to have a causal power. It was believed to be independent of test conditions and other methodological issues. However, ‘g’ or intelligence varies among time and place.

ANCOVA was devised to minimize pre-existing group differences (e.g. differences in SES). However, it is not possible to treat IQ as a covariate in neurocognitive research  since it is an attribute of the disorders (e.g. ADHD). A covariate should be used when the assignment to the independent variable is done randomly (1), the covariate is related to the outcome measure (2) and the covariate is unrelated to the independent variable (3). It can be used if the researcher is trying to find out the direct effect of the independent variable on the outcome variable and the covariate is spuriously related to either variable or when it mediates the relationship between the independent and the outcome variable.

The ANCOVA has several assumptions:

1. Homogeneity of regression
   The within-group regression of the covariate and the dependent variable are not different.
2. Normally distributed residuals
   The residuals should be normally distributed.
3. Equal variance
   The variance in each group should be equal.

The presence of a relationship between the covariate and the dependent variable does not imply that the covariate mediates or moderates the relationship between the group measure and the dependent variable.

There is no consensus on the definition of IQ. IQ can be used as a covariate for acquired brain damage if the preinjury IQ scores are available or when preinjury IQ proxies are available.