Margin of error

A margin of error value is displayed in various reports. The value is expressed as a percentage and represents a 95 percent confidence interval. What this means is that in 95 percent of cases (19 out of 20 times) a repetition of the same measurement will produce results within +/- the indicated number of percentage points. This value is based entirely on the size of a sample population and reflects only the statistical error that can be expected from the sample size. It does not take into account any effects caused by biased sample collection.