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All measures must be considered to be subject to error. We would have no way of knowing if, perchance, a measure turned out to be absolutely accurate. For example, the National Bureau of Standards has a ten gram weight (actually, because of manufacturing error just less than ten grams by about 400 micrograms, the weight of a grain or two of salt) (Freedman, Pisani & Purves, 1991). Despite the most careful weighing, done on a weekly basis, the values actually obtained for the standard weight vary by about 15 micrograms (one microgram is about the weight of a speck of dust) either way from the mean. Presumably, the mean of a long series of measures is the best estimate of the true weight of the ...

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