Mortality rate is a measure of the number of deaths (in general, or due to a specific cause) in some population, scaled to the size of that population, per unit time. Mortality rate is typically expressed in units of deaths per 1000 individuals per year; thus, a mortality rate of 9.5 in a population of 100,000 would mean 950 deaths per year in that entire population. It is distinct from morbidity rate, which refers to the number of individuals in poor health during a given time period (the prevalence rate) or the number who currently have that disease (the incidence rate), scaled to the size of the population.

One distinguishes:

In regard to the success or failure of medical treatment or procedures, one would also distinguish:

Note that the crude death rate as defined above and applied to a whole population can give a misleading impression. The crude death rate depends on the age (and gender) specific mortality rates and the age (and gender) distribution of the population. The number of deaths per 1000 people can be higher for developed nations than in less-developed countries, despite life expectancy being higher in developed countries due to standards of health being better. This happens because developed countries typically have a completely different population age distribution, with a much higher proportion of older people, due to both lower recent birth rates and lower mortality rates. A more complete picture of mortality is given by a life table which shows the mortality rate separately for each age. A life table is necessary to give a good estimate of life expectancy.

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