4. Current survivors
With current survivors we understand here the persons who were alive at the date when the information on deaths ends. This is always a year-end and therefore each year of age will correspond to one cohort.
The number of survivors has been determined on the basis of more or less independent sources such as permanent population registers, population censuses, official estimates and our own estimates derived from the deaths under the assumption that recent age-specific survival ratios in two or more successive cohorts have been identical or have changed linearly. The assumption of an identical survival ratio at a given age in successive cohorts was already used by DEPOID (1973). A report on the procedures as used in the Kannisto-Thatcher database is under preparation.
The experience with the assessment of the sources has been that good population registers produce the best and indeed very reliable population data up to the highest ages. However, countries which do not have them form the majority of the, database and include all the larger ones. The official population data for these, while reliable at most ages, are often unreliable and even implausible at ages approaching or exceeding 100 years. Reflecting this uncertainty they are given by many statistical offices only in a lump sum such as "90 and over". When the lump sum has been close to our "own estimate", we have accepted it and prorated it to individual ages according to the latter. If the two have been wide apart, we have accepted our own estimate on the grounds that, though not exact, it is consistent with the information on deaths and generally plausible. With younger ages our own estimates become increasingly uncertain while the official estimates tend to become more reliable. Therefore, as a rule, at a given age we have switched to the official figure. Quite often the switch has been made easy by the fact that the two series have come very close or even identical at some age around 95 or 90 years.
Updated by V. Castanova, 1 March 1999