When those being observed change their behaviours because they are reacting to the presence of an observer. Merely knowing that they are being observed can change how subjects behave, distorting their behaviour. +you hide the fact that you"re observing anything, by hiding, or blending in. +getting used to a stimulus to the point where it doesn"t interest you anymore. +present self multiple times doing the same behaviour so that they habituate ie: jane goodall. Presented self and hung out until they started ignoring her, then she started collecting her data. +can determine the most visited part of a mall by the wear and tear of the floor. There must be a significant association between 2 variables there must be temporal precedence, where the causal variable precedes or comes before the effect. There must be internal validity, meaning there are no alternate explanations for the observed relationship. It is a statistical relationship/association between two variables.