Errors in a research study due to the predisposed notions or beliefs of the experimenter.
Example: The results of the study are likely skewed since the researchers knew which patients were actually receiving the test drug and which were getting the placebo.
Grinnell, R. (2008). Experimenter Bias. Psych Central. Retrieved on May 23, 2015, from http://psychcentral.com/encyclopedia/2008/experimenter-bias/