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Source: Wikipedia:Margin_of_error

Calculating the margin of error for a survey

The standard error of a reported proportion or percentage p measures its accuracy, and is the estimated standard deviation of that percentage. It can be estimated from just p and the sample size, n, if n is small relative to the population size, using the following formula:

Standard error = [math]\sqrt{\frac{p(1-p)}{n}}[/math]

95% confidence interval = 1.96*standard error