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Mortality and morbidity
The study of mortality1 deals with the effect of death on the population. Occasionally the term mortality2 is used as a synonym for mortality rate2 or death rate2 which measures the frequency of deaths3. Where the expression mortality rate, or death rate is used without any qualifying adjective the crude death rate4 or general death rate4 is usually meant (cf. 135-8 for a general discussion of crude rates). This is generally an annual rate and consists of the ratio of the annual number of deaths occurring during a calendar year to the number exposed to the risk of dying during the same period. This number is equivalent to the mean population5 or average population5 for the period, and the population at the midpoint of the period can usually be substituted for the average population without appreciable error, if the size of the population is changing fairly uniformly. If the mortality of a sub-population (101-5) only is studied, we speak of specific death rates (cf. 134-6) among which sex-age-specific death rates6 are the most common. Age-specific death rates7 without distinction of sex are also used on occasions.
- 3. death n. — die v., — dead adj. syn. — deceased adj. or n.
Specific death rates may be used to study differential mortality1 or mortality differences1 between different groups. In general, men suffer an excess mortality2 as compared with women of the same age. Members of different occupations may also be subject to different rates of mortality (401-2). The study of differences in the general death rate of specific occupations is called the study of occupational mortality3. In a rather different sense the term occupational mortality4 may refer to mortality from an occupational disease5, i.e., a disease which is definitely associated with a particular occupation, e.g. pneumoconiosis among miners. Such mortality is sometimes called occupational disease mortality4.
Crude death rates (401-4) will depend on the structure (particularly the age structure (325-6)) of the population as well as on the level of mortality. If the mortality of different populations is to be compared, standardized mortality rates1 or adjusted mortality rates1 are sometimes computed to eliminate the effect of differences in population structure (144-4). Age is the characteristic for which mortality rates are adjusted most frequently by reference to a standard population2 with a given structure. If specific rates (134-6) for the population studied are available, it is possible to proceed by the direct method of standardization3 which consists of applying these rates to the corresponding groups of the standard population. The indirect method of standardization4 may be used to yield comparative mortality indices5. These are employed for the same purpose as standardized mortality rates. They are computed by applying standard mortality rates6 to the different groups of the population studied and summing these to obtain an expected number of deaths. Their value is obtained by comparing the observed deaths7 in the population with the expected deaths8 which would have occurred had the standard rates applied.
- 1. standardize v. — standardized adj. — standardization n., the process of standardizing.
- 5. If a crude death rate (401-4) is multiplied by a comparative mortality index, we obtain an indirectly standardized death rate. In British official terminology, when occupational mortality is studied, the figure obtained by direct standardization is called a comparative mortality figure, and that obtained by indirect a standardized mortality ratio.
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