Last Revised: January 30, 2007
but they are related to cohort death rates in a consistent way.^{9 }These probabilities of death are used to construct life tables.
Raw data versus estimates
Raw data (provided at the bottom of each country page) are the original data received from national statistical offices or other sources. The availability and format of these data varies somewhat across populations and over time. Therefore, it is important to note that the uniform data shown on country pages by single years of age (from age 0 to 109, with an open age interval for ages 110+) are sometimes estimates derived from aggregate data (e.g., fiveyear age groups, open age intervals such as 90+). As noted earlier, we frequently need to split the original data into finer age categories. In addition, the raw data often include persons of unknown age (in either death or census counts), which we redistribute (proportionally) across the age range. More information regarding the format of the raw data for a given population can be found in the Background and Documentation file (see Appendix), accessible from the country page.
Female / male / total
Raw data for women and men are always pooled when making calculations for the total population. Thus, “total” death rates and other quantities are not a simple average of the separate values for females and males, but rather a weighted average reflecting the relative size of the two groups at a given age and time.
Period versus cohort
Raw data are usually obtained in a period format (i.e., by the year of occurrence). In the HMD, most data are presented in a period format, but we also provide death rates and life tables in a cohort format (i.e., by year of birth) if the observation period is sufficiently long to justify such a presentation.
Period life tables
Whereas a cohort life table depicts the life history of a specific group of individuals (born in the same year or range of years), a period life table represents the mortality conditions at a specific moment in time. Observed period death rates are only one result of a random process for which other outcomes are possible as well. This inherent randomness is most noticeable at older ages where observed death rates may exhibit large fluctuations. Therefore, we smooth the observed values in order to obtain an improved representation of the underlying mortality conditions.^{10 }The smoothed death rates are used for subsequent calculations only above some very high ages. The cutoff age is based on a decision rule that involves both age and number of survivors in a particular population. As always, death rates (original at younger ages, smoothed above some age) are then converted to probabilities of death (q_{x}), and all remaining life table q u a n t i t i e s ( i . e . , l x , d x , L x , T x , e x ) a r e d e r i v e d f r o m t h e q x v a l u 11 e s .
Cohort life tables
Cohort life tables represent the mortality experience of the group of individuals who were born in the same year (or range of years for multiyear cohorts). We compute cohort
9 10 11
See pp. 3941 of the Methods Protocol for the formulas used to calculate cohort life tables. For details regarding the smoothing process, see pp. 3537 of the Methods Protocol. See pp. 3839 for the formulas used to derive life table quantities.

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