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I was wondering about when is it appropriate to use covariate in the analysis of data. Some cases are straightforward such as when you have pre – post intervention scores in homogeneous groups and use pre intervention scores as a covariate.

But, what to do in the case when comparing for example, motor skills in people with intellectual disability and those without IT. Does it make any sense to include IQ as a covariate when we know that these two groups are different in IQ scores? Any thoughts on this?