A ceiling or floor describes an uneven distribution at the high or low end of a range (respectively). There is a clustering of values with none above (for a ceiling) or none below (for a floor).
Significance of a ceiling or floor:
(1) The method does not discriminate between elements within the cluster. This may occur for a number of reasons, such as the test was too hard (for a floor) or too easy (for a ceiling).
(2) Analyzing the results with a method that assumes a normal distribution or a linear response will give misleading results.
(3) A change in the underlying object of interest may not be reflected by a change in its score.
Ways to approach a method showing a ceiling or floor effect:
(1) Redesign the method so that there is discrimination of the clustering elements.
(2) Select an appropriate method for analyzing the data.
(3) Do not use the original method as a means of following change over time.
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