This is a question that landed on my desk, and I don't have the requisite experience to reply.
A researcher has been asked to perform (by a journal reviewer) a two-way factorial ANCOVA on some microrarray data (a single array will be a single measurement of ~30,000 entities from a single sample in this case).
The suggestion was to use ethnicity and age as co-variables. I understand that ANCOVA requires at least one categorical and one continuous predictor variables.
The researcher has 3 ethnic groupings in his samples - 6 of group A, 2 of group B and 1 of group C. These samples are spread across three treatments (X, Y and Z), with 3 samples per treatment. This means the treatments cannot be split evenly by ethnicity, group C will only appear in one of the treatments.
Is this sample size/grouping issue a barrier to running a two-way factorial ANCOVA? Is there another test that might be more appropriate under the circumstances?