Not only do researchers need to be careful about whom they pick to be in groups in studies, but they also have to worry about how they solicit, record and interpret the data that they get from these subjects. Interviewer bias, as this problem is called, is more of an issue in medical studies when the interviewer knows the research subject's health status before questioning him or her.
A 2010 medical journal article on how to identify and avoid bias cites the hypothetical example of a study that's attempting to identify the risk factors for Buerger's disease, a rare disorder in which arteries and veins in the arms and legs become swollen and inflamed. If the interviewer already knows that a research subject has the disease, he or she is likely to probe more intensely for known risk factors, like smoking. So, the interviewer may ask people in the risk group, "Are you sure you've never smoked? Never? Not even once?"— while not subjecting patients in the control group to these kinds of questions [source: Pannucci and Wilkins].
An interviewer also can cause errant results in a study by giving subjects non-verbal cues when asking questions, such as with gestures or facial expressions, or tone of voice [source: Delgado, et al.].