How Typical Is the Typical Case?: Measuring Dispersion
Measures of central tendency provide a snapshot of the typical case; however, the same statistic may be obtained from samples or populations that are in fact quite dissimilar. For example, a sample of police recruits with a mean or median age of 23 is not likely to include people younger than 18 or older than 30, because most police departments have age requirements for incoming officers. A sample of offenders with a mean or median age of 23, however, will include offenders younger than 18 and much older than 30. In both these samples, the average person studied is 23 years old. But the sample of offenders will include more younger and older people than the sample of police recruits. The ages of the offenders are dispersed more widely around the average age.