That wouldn't call the results into question. It would, in fact, strengthen them. That's because the measured difference between the groups would be lower (bec

That wouldn't call the results into question. It would, in fact, strengthen them. That's because the measured difference between the groups would be lower (because the real difference would be lower if the groups are more alike than you think). Say you're testing a drug that's supposed to make people taller. You don't know it yet, but it really does make everyone grow 10cm overnight. You give it to half of your volunteers, and the other half gets placebo. The next day you find that the first group grew by 10cm compared to the control. Now say your grad student messed up and half of the control group also got the real thing instead of placebo. Those also grew by 10cm, making the average in the control group 5cm, and your treatment group's effect is suddenly lower.