I have two groups of subjects, each with scores on a memory task and on an IQ test. I would like to check whether there's a significant group difference in the memory measure while controlling for IQ.
I reckoned that for that, I need a 1-way between-subjects ANCOVA, with DV=memory score, predictor=group, and covariate=IQ. The Matlab command to achieve this appears to be aoctool. Following the example in its (rather incomplete) documentation , and having coded group as a categorical variable, I typed:
[h,a,c,s] = aoctool(PIQ, mem, group, 0.05, 'PIQ','mem','group','off','separate lines');
However, that just gives me the slopes&intercepts for the groups, which does not help me. What I need is a categorical comparison between the groups, rather than a regression between x vs y, as the aoctool example seems to put it.
My questions therefore are, how can I use aoctool to:
- obtain a p-value for the between-subjects effect of the group factor?
- scatter-plot the memory score as a function of IQ, with groups color-coded as in the documentation example? The above command does return a figure handle h, but it's an empty matrix (even in the case of the example dataset!), and I don't think it can be used to base a plot on it.