Mean = sum ÷ count
Median = middle value once sorted
Mode = the value that appears most often
SD = √( Σ(x − mean)² ÷ n ) population
= √( Σ(x − mean)² ÷ (n−1) ) sample
| Measure | Use it when |
|---|---|
| Mean | Values are evenly spread with no extremes |
| Median | A few very large or small values would distort the mean |
| Mode | You need the most common value, including for text-like data |
| Range | A quick sense of spread |
| Standard deviation | You need how tightly values cluster around the mean |
- Salaries are usually reported as a median — one very high earner drags the mean up and misrepresents the typical figure.
- Use the sample basis (n − 1) when your numbers are a sample of something larger, which is most of the time.
- In a normal distribution about 68% of values fall within one standard deviation of the mean, and 95% within two.