The terms ceiling effect and floor effect are opposites but they refer to the same phenomenon: the clustering of individual survey responses around a certain value. More specifically, ceiling effects occur when a considerable percentage of participants score the best or maximum possible score, while floor effects occur when the opposite happens, i.e.,  a considerable percentage of participants obtain the worst or minimum available score. This can be observed, for example, when a test is too easy (ceiling effect) or too difficult (floor effect). As a result, researchers can’t use the test to rank participants at either end of the scale.