The quartile deviation is one half of the difference between the upper quartile and the lower quartile in a distribution of scores. Thus, the upper quartile of any distribution of scores is the 75th percentile, that point below which are 75% of the scores. The lower quartile is the 25th percentile, that point below which are 25% of the scores. Calculation of the quartile deviation is done by subtracting the lower quartile from the upper quartile and then dividing the result by 2. If the quartile deviation is small, the scores are close together. If it is large, the scores are more spread out. The quartile deviation is the appropriate measure of variability when the data represent an ordinal scale.
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