selection bias

Selection bias is a distortion of evidence or data that arises from the way that the data are collected. It is sometimes referred to as the selection effect. The term selection bias most often refers to the distortion of a statistical analysis, due to the method of collecting samples. If the selection bias is not taken into account then any conclusions drawn may be wrong.
See more at Wikipedia.org...

An error in choosing the individuals or groups to take part in a study. Ideally, the subjects in a study should be very similar to one another and to the larger population from which they are drawn (for example, all individuals with the same disease or condition). If there are important differences, the results of the study may not be valid.