History--Specific events external to an experiment, but occurring at the same time, which may affect the criterion or response variable
•Maturation--Processes operating within the test units in an experiment as a function of the passage of time per se
•Testing--Contaminating effect in an experiment due to the fact that the process of experimentation itself affected the observed response
Main testing effect--the impact of a prior observation on a later observation
Interactive testing effect--the condition when a prior measurement affects the test unit’s response to the experimental variable