Whipple's index (or index of concentration) is a method to measure the tendency for individuals to inaccurately report their actual age or date of birth. Respondents to a census or other surveys sometimes report their age or date of birth to make it seem more culturally favorable, for example to appear younger, or to be born on a date that is considered luckier than their actual date of birth.
The index was invented by American demographer George Chandler Whipple (1866–1924) is applied to detect the extent to which age data show systematic heaping on certain ages as a result of digit preference or rounding. Typically the concern is for heaping on particular ages such as those ending in 0 and 5.
The index score is obtained by summing the number of persons in the age range 23 and 62 inclusive, who report ages ending in 0 and 5, dividing that sum by the total population between ages 23 and 62 years inclusive, and multiplying the result by 5. Restated as a percentage, index scores range between 100 (no preference for ages ending in 0 and 5) and 500 (all people reporting ages ending in 0 and 5).
The UN recommends a standard for measuring the age heaping using Whipple's Index as follows:
|Whipple's Index||Quality of Data||Deviation from Perfect|
|< 105||very accurate||< 5%|
|> 175||very bad||≥ 75%|
Although Whipple's index has been widely applied to test for age heaping, it assumes that the heaping is most likely to occur in 5 and 10 year intervals or some other fixed interval based on digit preference or rounding. While other measures of age heaping, such as Myer's Blended Index, can be applied to find preferences for any terminal digit, the patterns of heaping may be complex.
For example, it has been shown that among Han Chinese, age heaping occurs on a 12-year cycle, consistent with preferred animal years of the Chinese calendar. Whether this heaping represents actual fertility behavior (e.g., bearing children in favorable animal years) or selective memory or reporting of year of birth has not been determined. Although the heaping is not severe among Han, and it does not seem to be associated with age exaggeration, it is systematic and is higher among illiterate populations. On the other hand, among Turkic Muslim populations in China (Uyghurs and Kazakhs in Xinjiang Province) there is severe heaping at ages ending in 0 and 5; it is much higher among illiterate populations and appears to be correlated with age exaggeration. These traditionally Muslim nationalities do not use the Chinese calendar.
This finding suggests that use of Whipple's Index or other measures of age heaping that focus on specific digits or on decimal intervals of the age spikes may not be appropriate for all populations. In the case of China's 1990 census reported above, among Han heaping was found at ages 38, 50, 62, 74, and so on — ages that corresponded with being born in the Year of the Dragon. But among Turkic Muslims, heaping was found at ages 35, 40, 45, 50, 55, 60, and so on and increased in magnitude with age.
- Henry S. Shryock and Jacob S. Siegel, Methods and Materials of Demography (New York: Academic Press, 1976).
- See Shryock and Siegel, cited above.
- B. A. Anderson and B. D. Silver, “Ethnicity and Mortality in China,” in 1990 Population Census of China: Proceedings of an International Seminar (Beijing: State Statistical Bureau, 1994): 752–772; and B. A. Anderson and B. D. Silver, "Problems in Measuring Ethnic Differences in Mortality in Northern China," PSC Research Report No. 93–277, Population Studies Center, University of Michigan (Ann Arbor, MI, U.S.A.), April 1993.
- For another case of astrologically based cyclicality in China's birth rates, see Edward Cody, "Oh, to Be Born in the Year of the Pig," Washington Post (March 1, 2007).
- On the latter observation, based on the 1982 census of China, see A. J. Jowett and Y. Li, "Age-heaping: contrasting patterns from China," GeoJournal 28 (December 1992): 427–442.