Sample Planner
How many people do you need to ask?
For a class survey, a club poll or a newsroom questionnaire: pick the margin you can live with and see the sample it takes. Or work backwards from a margin a report printed.
Example plan
95% confidence · expected 50.0%
For ±3.0 points you need 1,068 completed answers.Halving the margin takes about four times the people.
| People asked | Margin | Width |
|---|---|---|
| 100 | ±9.8 | |
| 250 | ±6.2 | |
| 400 | ±4.9 | |
| 600 | ±4.0 | |
| 1,000 | ±3.1 | |
| 1,500 | ±2.5 | |
| 2,500 | ±2.0 |
Subgroups
A group that is part of the sample has its own, wider margin.
| Share of the sample | About | Margin |
|---|---|---|
| 50% | 534 people | ±4.2 |
| 25% | 267 people | ±6.0 |
| 10% | 107 people | ±9.5 |
A reported ±4.0 at 95% matches about 600 people (at a 50% share).If the report says it asked more people than that, its margin most likely includes the widening that weighting causes (the design effect).
Note: simple random sampling arithmetic, times the design effect you entered. Real surveys also lose precision to non-response and weighting. Why the population hardly matters.