Sample size to estimate a proportion with specified precision

Input Values

  

This utility calculates the sample size required to estimate a proportion (prevalence) with a specified level of confidence and precision.

Inputs are the assumed true value for the proportion, the desired level of confidence, the desired precision of the estimate and the size of the population for limited population sizes. The desired precision of the estimate (also sometimes called the allowable or acceptable error in the estimate) is half the width of the desired confidence interval. For example if you would like the confidence interval width to be about 0.1 (10%) you would enter a precision of +/- 0.05 (5%).

The program outputs the sample sizes required to estimate the true value with the desired precision and confidence, for both an infinite population and for a population of the specified size. If population size is left blank or zero, only the sample size for an infinite population is calculated.

Sample size is calculated using the formula:

n = (Z2 x P(1 - P))/e2

where Z = value from standard normal distribution corresponding to desired confidence level (Z=1.96 for 95% CI)
P is expected true proportion
e is desired precision (half desired CI width).
For small populations n can be adjusted so that n(adj) = (Nxn)/(N+n)

Estimated true proportion :
Confidence level :
Desired precision (+/-) :
Population size (for finite populations) :

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