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Dear Richie, apparently you are interested in experimental, treatment or disease effects in your data, and it accidentally happened that your treatment and control group were drawn from different subject pools. You will have to include age as a single covariate over both groups. However, this covariate will explain both age and experiment related variance (group differences) in your data and it will be impossible to distinguish between age and experiment effects. Even if there is any group difference left that is not explained by age it would not be a very reliable result. Best, Volkmar