Floor Effect

The floor effect is the mirror image of the ceiling effect: a measurement instrument has a lower limit, called the floor, below which it cannot register genuine differences between people who actually vary in their responses, so many scores bunch up near the minimum value the instrument allows. It shows up often in psychological testing, illustrated by giving an IQ test built for adults to preschool age children, most of whom would score near the test's minimum standard score of roughly 40, obscuring real variation in their intellectual ability; test developers address this by building age-appropriate assessments, and IQ tests intended for people with intellectual disabilities are deliberately written with simpler content and a lower floor so they can distinguish meaningfully among the people taking them. Because floor effects strip away variance at the low end of a scale, they threaten the validity of both experimental and correlational research, since an experimental manipulation can appear to have no effect purely because the outcome measure could not register any further decline.