Public health recommendations Intervention studies-► and interventions
Figure 13.1. Cascade from observational studies to public health intervention.
based on standardized death certificates has been legally mandated in many developed countries. The cause of death, in fact the underlying cause, is classified according to the International Classification of Disease (ICD). Revisions of the ICD, which are implemented about every 8 to 10 years, have to be considered in analysis of mortality trends. Further specific methodological issues, such as the accuracy of the cause of death statements on the certificate and problems with the use of a single cause of death have to be considered and are further discussed in standard textbooks (e.g., Lilienfeld and Stolley, 1994). In countries where a National Death Index has been established, computerized mortality data may be linked to other databases through some common identifier such as the Social Security Number. Mortality data for descriptive statistics also may be obtained in some countries from other sources such as autopsy, hospital, occupational, and financial records (e.g., insurance, pension funds), but these data sources may not represent the general population.
Basic measures of descriptive epidemiology include the incidence (which quantifies the occurrence of new diseases or functional impairments among persons at risk), the mortality (the occurrence of deaths), and the prevalence (the proportion of people with a certain disease or a certain functional impairment at a given point or at some point during a given period of time). The terms "risk" and "rate" are sometimes used synonymously, but strictly speaking they represent two different measures. Risk describes the probability that a person will develop a disease or die within a given time frame, and is usually derived from the proportion of those who develop the disease among a large group of people. Although the concept of risk is easily understood, it is almost impossible to measure risk over any appreciable time interval as some people inevitably either will be withdrawn from the study population due to competing risks such as dying from causes other than the outcome under study, or will be lost to follow-up. To overcome the problem of competing risks, a different measure of disease occurrence—the incidence (or mortality) rate—has been introduced and is widely used in epidemiology. A rate is calculated as the number of cases that occur in a study population, over the person time experienced by the population followed. Unlike risk, the rate measures not the proportion of the population that is affected, but rather the ratio of the number of cases to the time at risk for disease.
In addition, there are a large number of more complex measures derived from these basic measures, such as the (healthy) life expectancy, defined as the average length of (healthy) life from birth or from a given age, the years of life lost (with or without "quality adjustment''), the maximum lifespan, defined as the average maximum length of time people can be expected to survive, and so on.
For many chronic diseases and functional impairments, incidence, mortality and prevalence strongly increase with age, and they often vary by sex even within the same age groups. Therefore, it is often crucial that these measures are reported by age and sex, as so-called age and sex specific measures. However, in analyses of time trends or comparisons between populations, it is often desirable to come up with a single summary measure of disease (or functional impairment) frequency rather than a whole bunch of age- and sex-specific measures. On the other hand, it is essential that such comparisons are not biased by possible changes in the age structure of the population over time or between various populations to be compared. In such instances, calculation of age-standardized measures of incidence, mortality and prevalence may often be reasonable. With this procedure, the respective measures for the populations to be compared are recalculated assuming a common fixed age structure of some "standard population.'' Depending on the kind of study, the world population, or some regional or national population, may be used as standard.
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