Example
For 2, 4, 4, 6, 9, the sum is 25 and the count is 5, so the mean is 25 / 5 = 5. If one value changes from 9 to 90, the mean jumps even though most values did not change.
How the mean is calculated
The arithmetic mean adds all values and divides by the number of values. For 2, 4, 4, 6, 9, the sum is 25 and the count is 5, so the mean is 25 / 5 = 5.
Mean vs median vs average
In everyday language, average often means arithmetic mean. In statistics, average can also refer to other center measures such as median or mode. Mean uses every value, while median uses the middle after sorting.
How to interpret the result
The mean is useful when values are reasonably balanced around a center. Because every value contributes to the sum, one very high or very low value can move the mean substantially.
Common mistakes
- Forgetting to include zero values when zero is a real observation.
- Using the mean for skewed data without comparing the median.
- Mixing values measured in different units or time periods.
When mean is not the best summary
For income, property prices, response times or other skewed data, median can be a better single-number summary. Use standard deviation or range when the spread matters as much as the center.
Last reviewed: 2026-05-17