Standard Error of PERCENT Calculator

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Want to know how big a sample you'll need to get a desired degree of precision in measurements expressed as percents?

• Just enter either SAMPLE SIZE (n) or Target PERCENT (0-100) or BOTH.

• Then click on 'Compute'.

Standard Error of Target PERCENT will be calculated AUTOMATICALLY.

(By the way, this will only work if your BROWSER supports 'JavaScript'. If the numbers 100, 50, and 5 don't appear initially in the boxes, then you don't have 'JavaScript' support.)

The ERROR calculation means that when your study reports 'Target PERCENT', the 'real' underlying PERCENT will be within plus or minus the ERROR 66% of the time. Twice the ERROR will give a 95% confidence band, while three times the ERROR will give a 99% confidence band.

Notice that as sample size increases, error decreases. However, the relationship is not linear. Doubling the sample size will not cut the error in half. This is worth keeping in mind since sampling COSTS are essentially a linear function of sample size.

Note also that as the target percent gets either bigger or smaller than 50, the error decreases independently of sample size.