How to Determine the Sample Size for a Survey

A survey is, at best, an educated guess. Surveys can give you a good idea about what people think, but there is always some chance of inaccuracy or error. How accurate a survey is depends on the sample size. The more people you survey, the more precise the results.

Instructions

1. Determine what confidence interval you want. The confidence interval is the range within which your answer is accurate. For example, if you have a 5 percent confidence interval and 55 percent of your respondents answer yes, that means the answer is within 5 percent of what you got--between 50 percent and 60 percent of people in your target group agree with the statement.

2. Determine what confidence level you want. The confidence level is how sure you can be of your results. Most researchers use 95 percent, but you can use up to 99 percent. If you have a 95 percent confidence level and a 5 percent interval, you can be 95 percent sure your results are within 5 percent of what people actually believe.

3. Determine the population of your target demographic. If you were taking a survey about what males age 18 to 25 in Michigan think about extraterrestrials, your population would be equal to the number of 18- to 25-year-old males in Michigan. You can look up population information at the census bureau.

4. Go to a sample size calculator. Input your confidence level, confidence interval and your population, and press calculate. The calculator will tell you the sample size you need.

Tips & Warnings

• The better your confidence level and interval, the more people you will have to survey.

References

• Census Bureau
• Sample Size Calculator