Sample Size and Confidence Interval Calculator
This calculator will help you answer two questions.
- How many completed surveys do I need to have a reasonably accurate view of the entire population?
- How confident can I be that the information I collected is representative?
Before You Conduct a Survey
Follow these steps to determine how many completed surveys are necessary.
- Determine your population of interest (e.g., all customers, all female customers, all female customers in the eastern region).
- Estimate how many individuals are in this population; e.g. 20, 500 or 1,000,000.
- Determine how confident you need to be that your results are representative. A common rule of thumb is 95% confidence so that the results are accurate to within +/- 3%.
- Enter the above two numbers in the calculator.
- The result tells you how many completed surveys you need for each population. The "Uncorrected" confidence interval is for data that is not a continuous, normal distribution. Please note that these suggested sample sizes simply tell you if you have enough respondents to be representative of your population. This is not the same as statistical power which indicates if you have a large enough sample to determine differences between groups or identify relationships through correlations.
To determine how many surveys need to be distributed divide the above number by the anticipated response rate. For example, if you need 483 completed surveys and you anticipate a 30% response rate you need to mail 1,610 surveys (483/.3).
Sample size required if desired confidence interval and population size are known (p=.5)
Enter a number in the Error Desired and Population Size fields then tab out of those fields to calculate. Do not use commas or other formatting characters.
After You Have Conducted a Survey
Follow these steps to determine how confident you can be in the data you have collected.
- Determine your population of interest; e.g. all customers, all female customers, all female customers in the eastern region, etc.
- Estimate how many individuals are in this population e.g. 20, 500, 1,000,000.
- Enter the population size and your returned sample size in the calculator.
If you have a 3.5% confidence interval at the 95% confidence level the table is basically saying that if you conducted the same survey 100 times, 95 out of the 100 administrations should yield results within +/- 3.5% of the current number.
Confidence interval if population and sample size are known (p=.5)
Enter a number in the Population and Sample Size fields then tab out of those fields to calculate. Do not use commas or other formatting characters.
For more information:
Call: 1-800-447-3269
E-mail: info-ncs@pearson.com
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