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Mortality and morbidity
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401
The study of mortality^{1} deals with the effect of death on the population. Occasionally the term mortality^{2} is used as a synonym for mortality rate^{2} or death rate^{2} which measures the frequency of deaths^{3}. Where the expression mortality rate, or death rate is used without any qualifying adjective the crude death rate^{4} or general death rate^{4} is usually meant (cf. 1358 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 population^{5} or average population^{5} 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 subpopulation (1015) only is studied, we speak of specific death rates (cf. 1346) among which sexagespecific death rates^{6} are the most common. Agespecific death rates^{7} without distinction of sex are also used on occasions.
 3. death n. — die v., — dead adj. syn. — deceased adj. or n.
402
Specific death rates may be used to study differential mortality^{1} or mortality differences^{1} between different groups. In general, men suffer an excess mortality^{2} as compared with women of the same age. Members of different occupations may also be subject to different rates of mortality (4012). The study of differences in the general death rate of specific occupations is called the study of occupational mortality^{3}. In a rather different sense the term occupational mortality^{4} may refer to mortality from an occupational disease^{5}, i.e., a disease which is definitely associated with a particular occupation, e.g. pneumoconiosis among miners. Such mortality is sometimes called occupational disease mortality^{4}.
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Crude death rates (4014) will depend on the structure (particularly the age structure (3256)) of the population as well as on the level of mortality. If the mortality of different populations is to be compared, standardized mortality rates^{1} or adjusted mortality rates^{1} are sometimes computed to eliminate the effect of differences in population structure (1444). Age is the characteristic for which mortality rates are adjusted most frequently by reference to a standard population^{2} with a given structure. If specific rates (1346) for the population studied are available, it is possible to proceed by the direct method of standardization^{3} which consists of applying these rates to the corresponding groups of the standard population. The indirect method of standardization^{4} may be used to yield comparative mortality indices^{5}. These are employed for the same purpose as standardized mortality rates. They are computed by applying standard mortality rates^{6} 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 deaths^{7} in the population with the expected deaths^{8} 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 (4014) 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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