Individual Variability:
The other problem is that it is impossible to maintain homogeneity across the groups; this method uses individuals, with all of their subtle differences, and this can skew data.
Age, gender and social class are just some of the obvious factors but intelligence, emotive quotient and every other personality construct can influence the data. If, for example, you were using a between subjects design to measure intelligence, how do you guarantee that emotion does not play a role? Some people may be very intelligent but are nervous when completing tests, so achieve lower scores than they should. These individual differences can create a lot of background noise, reducing the effects of the statistics and obscuring genuine patterns and trends.