A margin of error is the half-width added to and subtracted from an estimate to form a confidence interval at a stated level. An estimate of 44 percent with a margin of plus or minus four percentage points is commonly displayed as an interval from 40 to 48 percent. The confidence level describes the long-run success of intervals produced by the method, not certainty about each respondent or a promise that this one estimate is correct.

Sampling error is only one part of total survey error. A margin does not measure omitted groups, self-selection, systematic nonresponse, leading language, inaccurate answers, interviewer pressure, processing mistakes, or opinion change after the interviews. If a biased frame excludes a distinctive group, increasing the number selected from that same frame can narrow the interval around a systematically mistaken estimate.

Subgroups also contain fewer cases than the full sample. A margin reported for 1,500 adults should not be attached unchanged to an estimate based on 180 adults in one age group.

Read every margin conditionally: it describes random variation if the sampling assumptions hold. Then inspect the rest of the chain.

Video lesson: Science Behind the News: Opinion Polls & Random Sampling