7.4 Conclusion

Age, period, and cohort effects are important ways to understand changes in society over time. People most commonly mix up period and cohort effects, such as offering cohort explanations for changes in period measures, or mistaking one kind of change for the other. In many cases, period and cohort rates tell the same overall story. Where they tend to diverge is when there are large societal changes within short periods of time (like in Singapore), as we have seen in the case of life expectancy. Under such circumstances, period measures are usually bad estimates of cohort change.

We often assume that experts and social scientists quoted in newspapers are making statements about societal change based off empirical evidence and research that they have done. But sometimes experts may just be speculating about something there is no real data to show for (e.g., no cohort data, and no research done to project cohort trends). In such a case (and with the application of some common sense), anyone’s speculation may be as good as theirs. Where these speculations happily coincide with reality they may be of some use, but readers should carefully evaluate whether it is suitable for certain numbers to be used to make certain points.