Bias
In research, bias is a flaw in the design of a study that affects the results. For example, there is selection bias, which occurs when study subjects are chosen in a way that can misleadingly increase or decrease the strength of an association. For example, women are often underrepresented as subjects in studies, a fact that may result in biased results.

Bias may occur in other ways, too. If the questions in a study are leading ("Should we all be free to choose our own doctors?" as opposed to "How important is being able to choose a physician?"), this will affect the way people answer. Avoiding bias is one reason why scientists prefer double-blind studies.