How to find the sample size you need
- Choose the confidence level, usually 95%, and the margin of error you can accept, such as 5%.
- Leave the expected proportion at 50% unless you have an earlier estimate: 50% gives the safest, largest sample.
- Enter the population size if it is small (a school, a company); leave it empty for a large population.
Margin of error and sample size
At 95% confidence and p = 50%:
| Margin of error | Sample size |
|---|---|
| 10% | 97 |
| 5% | 385 |
| 3% | 1,068 |
| 2% | 2,401 |
| 1% | 9,604 |
Halving the margin of error needs four times the sample. That is why most polls settle around 1,000 responses for about ±3%.
What it doesn’t cover
The formula assumes a random sample. A large but biased sample, such as only the people who chose to answer an online poll, can still be badly wrong.