threshold effect
an effect in a dependent variable that does not occur until a certain level, or threshold, is reached in an independent variable. For example, a drug may have no effect at all until a certain dosage level (the threshold value) is reached. The change in the dependent variable can be an irreversible dichotomous change (e.g., from functional to not functional), or it can be continuous, such that the dependent variable changes each time the independent variable crosses the threshold in either direction.