12. Point estimation
A point estimate is a single value—a “point"—taken from a sample and used to estimate the corresponding parameter in the population. A statistic is an estimate of a parameter: X estimates m, s estimates s, s2 estimates s2, r estimates r, and P estimates p.
Opinion polls offer the most familiar example of a point estimate. When, on the eve of a presidential election, you hear on CNN that 55% of voters prefer Candidate X (based on a random sample of likely voters), you have been given a point estimate of voter preference in the population.
Point estimates should not be stated alone. That is, they should not be reported without some allowance for error due to sampling variation. Without additional information, it cannot be known whether a point estimate is likely to be fairly close to the mark (the parameter) or has a good chance of being far off.
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