Clustering at the upper limit of a measurement instrument can prevent it from distinguishing higher underlying levels. A bounded score need not exhibit a ceiling effect, and a sample maximum alone does not establish one. The issue concerns what variation the instrument can measure, rather than a proof that a treatment has no effect.
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The ceiling effect in statistics refers to a situation where a measurement instrument or scale has an upper limit that restricts the ability to measure higher values effectively. This often results in a clustering of scores at the high end of the scale, which can limit the variability of the data and impact the validity of conclusions drawn from the analysis.