20.8.1 Significant Digits
The use of a numeral in a numbers column (eg, the ones column) implies that the method of measurement is accurate to that level of precision. For example, when a reporter attempts to estimate the size of a crowd, the estimate might be to the nearest tens of number of people, but would not be expressed as an exact number, such as 86, unless each individual was counted. Similarly, when an author provides a number with numerals to the right of the decimal point, the numerals imply that the measurement used to obtain the number is accurate to the last place a numeral is shown. Therefore, numbers should be rounded to reflect the precision of the instrument or measurement; for example, for a scale accurate to 0.1 kg, a weight should be expressed as 75.2 kg, not 75.23 kg. Similarly, the instrument used to measure a concentration is accurate only to a given fraction of the concentration, for example, 15.6 mg/L, not 15.638 mg/L (see Table 2 in 18.5.10, Units of Measure, Conventional Units and SI Units in JAMA and the Archives Journals, Laboratory Values, for the appropriate number of significant digits). Numbers that result from calculations, such as means and SDs, should be expressed to no more than 1 significant digit beyond the accuracy of the instrument. Thus, the mean (SD) of weights of individuals weighed on a scale accurate to 0.1 kg should be expressed as 62.45 (4.13) kg. Adult age is reported rounded to 1-year increments, so the mean could be expressed as, for example, 47.7 years.