Mohammed T. Abou-Saleh

NORMAL AGEING—A PROBLEMATICAL CONCEPT 69obligation to carry out exploratory data analyses to ensure thatthe effect that is to be neglected is in fact, statistically unimportant,should not need emphasis.The inevitable confound between age, cohort and period effectsis awkward because it means that we cannot use this mosteconomical of all research strategies to solve all of the manyproblems connected with ageing. However, bearing their overridinglimitation in mind, it must be stressed that cohort analysesare not merely an economical, but methodologically flawed,substitute for laborious longitudinal studies. They rather providea means by which we can answer classes of questions that cannotbe approached in any other way. The fact that the cohorts neednot contain the same individuals is a statistical advantage as wellas logistical convenience. For example, it can be very useful tosample quite different populations of the same birth cohort atdifferent times in their lives so as to make an intracohort trendstudy. There are also unique advantages in comparing n quitedifferent age samples of a population recruited at successiveperiods of time. This allows us to draw up what has becomeknown as a ‘‘standard cohort table’’, in which the different agecohorts are ordered in rows, and the successive years in whichthese different age groups were sampled are ordered in columns.This allows us, at least, a rapid and convenient way to carry outexploratory analyses of our data. We can pick out intracohorttrends, which become apparent by scanning down the diagonals;that is, we can see how members of a particular age group(whether represented by the same individuals followed over time,or by quite different samples) are affected by both the passage oftime (and so, among other factors, by their own biological ageing)and also by changes in the social, economic or epidemiologicalcontexts in which they were studied. Within each of thesesampling periods we can compare age cohort differences byscanning down the columns. Finally, by scanning along the rows,we can examine period effects which occur as one age cohortreplaces another.No single technique of comparison, whether cohort analysis,longitudinal analysis or even cross-sequential analysis, canprovide a universal methodological panacea. Rather, each cansolve problems that the others cannot approach, and none save usthe effort of carefully thinking through the questions that we wishto ask of our data, and intelligently considering the implicationsof the comparisons we must make to answer them. To study theeffect of ageing we must simultaneously acquire both crosssectionaland multiple longitudinal data and interpret them assensibly as we can.REFERENCES1. Costa PT, McCrae RR. An approach to the attribution of aging,period and cohort effects. Psychol Bull 1982; 92: 238–50.2. Palmore E. When can age, period and cohort be separated? Soc Forces1978; 57: 282–95.