Estimate the number of independent responses needed for a target margin of error in a proportion.
Sample Size Calculator
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Sample Size
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Table of Contents
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What is Sample Size?
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How to Use This Calculator
- Enter or select Confidence level, Margin of error, Proportion estimate, Population.
- Check the unit beside each measurement. For a percentage or rate, use the scale and time period stated in the form.
- Choose Calculate, then compare the output with the formula and example below. If you change a value or option, calculate again to update the result.
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Formula Used
n₀=z²p(1−p)/e²; finite correction: n=n₀/(1+(n₀−1)/N)
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Variables Explained
| Input | What it means |
|---|---|
| Confidence level | The coverage level used to choose the normal-distribution critical value. |
| Margin of error | Use the measured or stated value for this item, with the unit or category shown beside the field. |
| Proportion estimate | Use the measured or stated value for this item, with the unit or category shown beside the field. |
| Population | Total number of eligible members in the population being studied. |
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Worked Example
At 95% confidence, p=0.5 and e=0.05 give about 385 before finite-population correction.
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Why Use This Calculator?
Plan response counts under a stated precision target before collecting data.
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Common Mistakes
- A larger sample does not remove selection bias. Round the required count upward.
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Frequently Asked Questions
Does a calculated statistic prove a conclusion?
A statistic describes the supplied data under stated assumptions. Sampling design, missing observations and the suitability of the model still determine what can reasonably be concluded.
Should I include every observation?
Use the observations required by your study design and state any exclusion rule. Removing values only because they change the answer can bias the analysis.
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Sources & References
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