The IDL contains thoroughly validated information on individuals of extreme ages.
Data collection was performed in such a way that age-ascertainment bias is avoided.
Because most reports of reputed supercentenarians are erroneous, age validation is
essential to compilation of accurate data. The methods used for verifying age differ
among countries. In view of these differences, the IDL provides information about how
meticulously each case has been validated, what documents were provided to the IDL
in the course of the validation process, and the name of the researcher responsible for the validation.
The variable VALIDATION assigns the validation level of a case to one of two
categories. VALIDATION=A indicates full validation and requires that an early life
document, preferably a birth record or baptism record, or an early census record, was
available and has been checked. In the same way, the date of death of a fully validated
case must also be backed up by a document, such as a death certificate or an entry
in a death index.
For several countries, the validation procedure is not documented as thoroughly as full
validation requires, but individual cases have been carefully checked. Such cases are
tagged as VALIDATION=B. They can arise, for example, when a local residence register
officially confirms a date of death, but does not issue any personal document on the
individual. There may also be cases in which the sequence of entries in national
censuses had been carefully checked by the respective national statistical office over
the course of the individual's life.
Age ascertainment bias
The IDL emphasizes that data on validated supercentenarians are free of age
ascertainment bias. This is important because some kinds of identification procedures
may register people who are, e.g. 115, with greater probability than people who are
merely 110. For example, media coverage is more common for the oldest cases, thus
individuals who died at younger ages (i.e. shortly after their 110th birthday) may be
underrepresented in the press. Such an age bias in the inclusion of supercentenarians
in a database can result in serious misestimates of mortality patterns. Consequently,
the IDL aims to compile lists of validated supercentarians in which the probability
of identifying cases is age-independent.