Ceiling effect
A ceiling effect is when a measurement tool used in an evaluation has an upper limit that is too low to capture the full range of participant outcomes, resulting in a clustering of scores at or near the highest possible value. In practice, this means that the instrument cannot distinguish between high-performers or detect improvement over time, because many responses have already 'maxed out' the scale.
Ceiling effects limit the ability to observe change, compare groups, or accurately estimate program impact when participant begin near or quickly reach the top of the measurement scale.