# Sample Size Calculator

This calculator allows you to determine an appropriate sample size for your study, given different combinations of confidence, precision and variability.

For large populations, it uses Cochran's equation to perform the calculation.

$x = {Z^2pq \over e^2}$

For small populations of a known size, it uses Cochran's equation together with a population correction to calculate sample size.

$n = {n_0 \over 1 + {{(n_0 - 1)} \over N}}$

Instructions

The default values we provide below will work well for many scenarios.

Precision Level is the margin of error you're prepared to tolerate - e.g., 5% means a result that is within 5 percentage points of the true population value.

Confidence Level is a measure of confidence in the precision of the result. For example, selecting 5% as the level of precision, and 95% as the confidence level, indicates a result that is within 5% of the real population value 95% of the time.

Estimated Proportion is a measure of variability. We suggest you leave this at 0.5 - maximum variability - unless you have prior knowledge about the population from which you are drawing your sample.

The final thing to note is that if you know the size of the population from which you wish to take a sample, you can select the Small Population option, and specify population size. This will result in a smaller sample.

The Calculator

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