The mean is the arithmetic average of a set of numbers. It is calculated by summing all the values and dividing by the number of values.

In a distribution of test scores, the mean provides a single number that represents the central tendency of the group. It is sensitive to every score, including extreme values. A single very high or very low score can pull the mean upward or downward, which is why the median is sometimes preferred when outliers are present. The mean is the most commonly reported measure of central tendency in psychological research and is the foundation for many statistical tests.

When scores are distributed symmetrically, the mean, median, and mode coincide. In skewed distributions they diverge. Researchers therefore examine the shape of the data before deciding which measure best summarizes it. The mean also serves as the reference point for calculating variance and standard deviation.

Adding the numbers and dividing by how many there are yields the mean. That simple operation remains the workhorse of descriptive statistics.