A sample proportion or a sample mean estimates the same number for the whole group, but every random sample lands a little differently. Simulating many random samples shows how far the estimates spread, and two standard deviations of that spread is the margin of error. The crew's rule for a proportion is 2√(p(1 - p)/n). The estimate plus and minus the margin is an interval for the whole group. A bigger sample gives a smaller margin, four times the sample halves it, and a report without a margin or a sample size cannot be judged.