However, theoretically we have reasons to use these groups of 20% and 80% because the highest scoring students may be cognitively gifted.

That would mean, those in the 79th percentile have to be treated the same as identical to those in the 1st percentile.

Plus, you did not mention where your sample comes from. If one uses normative values to determine who is gifted

and who is not -- then o.k., that would be dubious (see above), but one could see some reason in it.

But if one determines giftedness just by dichotomizing a

*sample*, so that ANY sample contains 20% "gifted", then this is

obviously misleading.

Anyway, I just mentioned this for people who might by chance be directed to this thread.

My supervisor advised me to conduct ANOVAs. So to use the two groups of 20% and 80% as a dependent variable, I need to compute a new variable for the testscore, controlling for age. Do you know if this is possible?

I simply do not know the research question behind that. My guess:

a) tell your supervisor that you can conduct a linear regression with "age" and "giftedness yes/no" as predictors; this means, the effect of giftedness is adjusted for age

b) tell your supervisor, that you can conduct an ANCOVA with "giftedness" as factor, and "age" as covariate; this means, the effect of giftedness is adjusted for age; you can display "marginal means" (adjusted for age) for the 2 groups

c) ask your supervisor what

exactly he wants you to do, and why,

if a) and b) do not satisfy him.

With kind regards

Karabiner