Descriptive statistics summarize and organize research data. Measures of central tendency represent the typical score in a set of scores. The mode is the most frequently occurring score, the median is the middlemost score, and the mean is the arithmetic average of the set of scores. Measures of variability represent the degree of dispersion of scores. The range is the difference between the highest and lowest scores. The variance is the average of the squared deviations from the mean of the set of scores. And the standard deviation is the square root of the variance.

Many kinds of measurements assume a normal, or bell-shaped, curve when graphed. A certain percentage of scores fall below each point on the abscissa of the normal curve. Percentiles identify the percentage of scores that fall below a particular score.