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Mortality
Mortality refers to death as a study endpoint or outcome. Broader aspects of the study of death and dying are embraced in the term thanatology. Survival is an antonym for mortality. Mortality may be an outcome variable in populations or samples, associated with treatments or risk factors. It may be a confounder of other outcomes due to resultant missing data or to biases induced when attrition due to death results in structural changes in a sample. Mortality is an event that establishes a metric for the end of the life span. Time to death is frequently used as an outcome and, less frequently, as a predictor variable. This entry discusses the use and analysis of mortality data in research studies.
Population Mortality Rates
Nearly all governments maintain records of deaths. Thus many studies of mortality are based on populations rather than samples. The most common index of death in a specific group is its mortality rate. Interpretation of a mortality rate requires definition of the time, causes of death, and groups involved. Mortality rates are usually specified as the number of deaths in a year per 1,000 individuals, or in circumstances where mortality is rarer, per 100,000 individuals. A mortality rate may be cause specific, that is, refer to death due to a single condition, such as a disease or type of event or exposure. All-cause mortality refers to all deaths regardless of their cause. Mortality rates are often calculated for whole populations but can be expected to vary as a function of demographic variables, notably sex and age. Important subgroup mortality rates, as recognized by the World Health Organization, include the neonatal mortality rate, or deaths during the first 28 days of life per 1,000 live births; the infant mortality rate, or the probability of a child born in a specific year or period dying before reaching the age of 1 year; and the maternal mortality rate, or the number of maternal deaths due to childbearing per 100,000 live births. The adult mortality rate refers to death rate between 15 and 60 years of age. Age-specific mortality rates refer to the number of deaths in a year (per 100,000 individuals) for individuals of a certain age bracket. In comparing mortality rates between groups, age and other demographics must be borne in mind. Mortality rates may be standardized to adjust for differences in the age distributions of populations.
Use in Research Studies
Mortality and survival are central outcomes in a variety of research settings.
Clinical Trials
In clinical trials studying treatments for life-threatening illnesses, survival rate is often the primary outcome measure. Survival rate is evaluated as 1 minus the corresponding mortality rate. Randomized controlled trials are used to compare survival rates in patients receiving a new treatment to that in patients receiving a standard or placebo treatment. The latter is commonly known as the control group. Such trials should be designed to recruit sufficient numbers of patients and to follow them for long enough to observe deaths likely to occur due to the illness to ensure adequate statistical power to detect differences in rates.
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