Use the population formula when your data is the entire group, and the sample formula when it is a subset used to estimate a larger group. The sample formula divides by n minus 1.
Side by side
| Population | Sample | |
|---|---|---|
| Divides by | n | n – 1 |
| Use when | You have every value | You have a subset |
| Result | Slightly smaller | Slightly larger |
| Example 2,4,4,4,5,5,7,9 | 2.00 | 2.14 |
When to use which
Most real-world data is a sample, so the sample formula is the safe default in statistics classes unless the question says the data is the whole population.