What is bias?
bias is In general, any factor that distorts the true nature of an event or observation. In clinical observations, a bias is any systematic factor other than the intervention of interest that affects the magnitude of an observed difference (i.e. trends to increase or decrease) in the outcomes of a treatment group and control group. Bias diminishes the accuracy (though not necessarily the precision) of an observation. Randomization is a technique used to decrease this form of bias. Bias also refers to a prejudiced or partial viewpoint that would affect someone’s interpretation of a problem. Double-blinding is a technique used to decrease this type of bias.
source: A GLOSSARY OF TERMS FOR COMMUNITY HEALTH CARE AND SERVICES FOR OLDER PERSONS. World Health Organization, 2004.