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Multilingual Demographic Dictionary, second unified edition, English volume

Cohort measure

Multilingual Demographic Dictionary, second unified edition, English vol.
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Cohort measure  (COHORT measure)

The primary objective of cohort analysis (103-4) is the study of the intensity 1 and tempo 2 or timing 2 of demographic phenomena. The intensity of a phenomenon initiated by one non-renewable event (201-4) may be measured by either the ultimate frequency 3 of occurrence for the given event or by its complement. The ultimate frequency reflects the proportion of persons who would have experienced the event, in the absence of extraneous influences, during the existence of the cohort (116-2). The intensity of a phenomenon initiated by a renewable event (201-5) such as births or migratory moves, can be measured by the mean number of events 4 per person in the cohort, also in the absence of extraneous influences. Tempo or timing may be defined as the distribution over time within the cohort of the demographic events corresponding to the investigated phenomenon. The results of cross-sectional analysis or period analysis (103-5) are summarized by period measures 5 — as opposed to cohort measures 6 — which can be constructed in various fashions. A commonly used technique consists in attributing the observed rates pertaining to various ages or durations to a hypothetical cohort 7 or synthetic cohort 7 .

  • 3. This ultimate frequency or its complement has received various names according to the phenomenon studied: parity progression ratio (637-7), frequency of definitive celibacy (521-1) ... It is best not to use the word proportion as part of these names, and to reserve it for observed proportions. For instance, the frequency of definitive celibacy must be kept distinct from the proportion single at a given age, as recorded in a census.
  • 4. It is not unusual to give the same name to the observed mean number of events per person, and to the number that would have been observed in the absence of extraneous influences such as mortality. Distinct phrases should be used; for instance, the number of children ever born (637-2) can be distinguished from cumulative fertility (636-2).
  • 5. Because cross-sectional analysis and hypothetical cohorts were used before genuine cohort analysis, the names of period indices often seem to imply that they refer to a cohort. This usage may lead to apparent contradictions. For example, parity-specific birth probabilities may exceed one for certain years when many postponed births are made up.