Occurs when differences between groups are present at the beginning of the experiment.
How is selection bias controlled?
This bias is controlled by random assignment, assigning the subjects to each group based on chance rather than human decision.
Involves the influencing of performance due to the subject’s belief about the results. In other words, if I believe the new medication will help me feel better, I may feel better even if the new medication is only a sugar pill. This demonstrates the power of the mind to change a person’s perceptions of reality.
How is Placebo Effect controlled?
To control this bias, subjects are often not informed of the purpose of the experiment. This is called a blind study.
The same way a person’s belief’s can influence his or her perception, so can the belief of the experimenter. If I’m doing an experiment, and really believe my treatment works, or I really want the treatment to work because it will mean big bucks for me, I might behave in a manner that will influence the subject.
How is Expirementer Bias controlled?
This bias is controlled by using a double blind study, meaning both the experimenter and the subjects are blind to the purpose and anticipated results of the study.
Psychology: Chapter 1, Section 3--Research Biases
Different types of research biases and ways to control them.