Population censuses are powerful resources for studying long term social and economic change. A single census may provide retrospective estimates of socioeconomic variables for 50-75 years prior to the census. Multiple decennial censuses may provide century long series, providing at the same time a test of data quality. Historical estimates produced in this way are frequently unavailable from any other source, especially for developing countries.
Norman Ryder observed half a century ago that cohort total fertility may be identified with period total fertility at the time the cohort reaches its mean age at childbearing. The immense but under appreciated practical value of this observation will be illustrated with numerous examples.
Ryder's observation generalizes to any class of events experienced by members of a birth cohort. Literacy data from the 1981-2001 censuses of Portugal shows the development of literacy over a century--with gender differences. A closely related technique, time-plotting age-specific population growth rates show long term fluctuations in population change. Applied to three or more population censuses, it is a powerful tool for assessing the quality of age distribution data.
A striking characteristic of these methods is how simple they are to learn and apply. Few if any other demographic techniques provide such powerful results in return for such modest investment.
Griffith Feeney is a demographer and statistician. His first published paper appeared in Demography in 1970 when he was a graduate student in the Department of Demography at the University of California at Berkeley. On completing graduate work, he went to the East-West Center in Honolulu, Hawaii, where he spent the next 26 years alternating between research, practical work in national statistical offices throughout Asia, and teaching graduate students and mid-career professionals. Since 1998 he has been an independent consultant in demography and statistics working mainly in developing countries in Africa and Asia. Most recently, he spent four months in Zambia working at the CSO as DFID Census Logistics Advisor. He may be contacted by email at <firstname.lastname@example.org>