Ceiling effect in statistics and psychology means that all of the test participants achieve the maximum possible (or very near to the maximum) score on a test, that is, the scores of the test participants are all crowded near the highest achievable value (the “ceiling”).
This can happen if the test is too easy. In such a case, all those who give the test achieve high scores and it is not possible to rank them in order of their ability. Thus the entire point of the test, which was to sort the candidates according to their ability, has been defeated because of the ceiling effect. This explains why the ceiling effect is problematic.
Ceiling Effect vs Floor Effect:
As opposed to the ceiling effect, the floor effect is when the scores of the candidates are crowded near the lower end of the spectrum. This can be because the test is too hard and therefore none of the candidates are able to achieve good scores.
The floor effect can also happen if the candidates are not given sufficient time to complete the test resulting in low scores. The floor effect is problematic for the same reason as the ceiling effect; it makes us unable to sort candidates according to their ability.
Ceiling Effect in Medicine and Pharmacology:
The ceiling effect in medicine and pharmacology refers to the fact that many medicines have a bound on their effectiveness and increasing the dosage of the medicine beyond a certain point does not have any appreciable effects on the body. This means that the effectiveness of the medicine has hit a “ceiling”.
How can you detect the ceiling effect?
We can detect the ceiling effect visually by simply drawing a scatterplot and observing whether the values crowd near the maximum limit. Another way would be to conclude the ceiling effect if a certain proportion (say 95%) of the observed values lie above a specific pre-decided limit.