A ‘margin of error’ is often reported with opinion polls using a phrase such as ‘the proportion intending to vote for the Red party is 35%, with a margin of error of 3%.’ This actually means that on an average of 19 times out of 20, the sample proportion obtained will be within 3 percentage points of the true value of the population proportion; it is therefore defining a 95% confidence interval for the population proportion, with the margin of error amounting to half the width of that interval.