an element that affects the results of an observation or experiment —note 1. the term is used particularly to describe a set of related treatments in an experiment; each treatment of the set is a different state or level of the factor, e.g., different varieties of the same plant species, different levels of application of a fertilizer, different methods of cultivation —note 2. the randomization of an experimental design is undertaken to separate the effect of these controlled factors from the effects of the uncontrolled factors that result in experimental error This definition last updated 08/07/2008