Floor Effect / Basement Effect: Definition

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The floor effect is what happens when there is an artificial lower limit, below which data levels can’t be measured. Usually, this is because of inherent weaknesses in the measuring devices or the measurement/scoring system.

The lower limit, which affects dependent variables, is referred to as the floor, and can badly skew a data distribution if not accounted for. For example, an exam might simply be too tough, giving an unusually large number of people very low scores; If you know that a particular exam out of a set is very tough, you could use a weighted mean (giving the tough exam more weight).

The floor effect is also sometimes called the basement effect.

Examples of the Floor Effect

A simple example of a floor effect might be found in scores of a mathematics test given to a set of incoming freshmen at a college. Suppose this test consists of five difficult math problems. If only a small amount of the incoming freshmen are capable of making any attempt at the questions, the test has a very high floor and is not useful in differentiating the majority of the students.

Now suppose all the questions on it were tough, perhaps higher-level statistics. Then it would not be useful in separating those who failed high school math from those who aced Calculus, as all students would probably end up with an extremely low score.

A test which included a few questions in Calculus might be successful in identifying those who took honors math, but the high floor would still make it useless in identifying those students who never went higher than advanced mathematics but were very good in everything they undertook.

See also: Ceiling Effect


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  • Howe, Adele. How to Detect Ceiling and Floor Effects. Retrieved from
    https://www.cs.colostate.edu/~howe/EMAI/ch3/node8.html on June 14, 2018
  • Orsini, Pezzuti, & Hulbert. Beyond the floor effect on the Wechsler Intelligence Scale for Children–4th Ed. (WISC-IV): calculating IQ and Indexes of subjects presenting a floored pattern of results. Retrieved from J Intellect Disabil Res. 2015 May;59(5):468-73. doi: 10.1111/jir.12150 on June 14, 2018.
  • Floor Effect: Quick Reference. Retrieved from http://www.oxfordreference.com/view/10.1093/oi/authority.20111105222543882 on June 14, 2018.

Stephanie Glen. "Floor Effect / Basement Effect: Definition" From StatisticsHowTo.com: Elementary Statistics for the rest of us! https://www.statisticshowto.com/floor-effect/

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