Relationship Among Prevalence, Incidence Rate, and Average Duration of Disease
(cross-sectional and longitudinal) examining the relationship between being religious/spiritual and the prevalence of depressive symptoms and depression. Community Dent Health. Dec;23(4) The relationship between prevalence and incidence of dental caries. Some observational consequences. Table 27 identifies the prevalence and incidence rates for drug offenses in the the linear relationship between IQ and lifetime prevalence is also negative for.
Develop the outcome under investigation Refuse to continue to participate in the study Migrate Die Enter the study some time after it starts To account for these variations during follow up, a more precise measure can be calculated, the incidence rate. Incidence Rate Incidence rates also measure the frequency of new cases of disease in a population.
However, incidence rates take into account the sum of the time that each person remained under observation and at risk of developing the outcome under investigation.
Calculation of person-time at risk The denominator in an incidence rate is the sum of each individual's time at risk and is commonly expressed in person years at risk.
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The incidence rate is the rate of contracting the disease among those still at risk. When a study subject develops the disease, dies or leaves the study they are no longer at risk and will no longer contribute person-time units at risk.
Person-time at risk Person-time at risk is a measure of the number of persons at risk during the given time-period. In the graph below, different numbers of persons are at risk N-d during the time-period t. The total person-time at risk is represented by the area below the line Y.
Persons who have developed the disease d are no longer considered at risk as they already have the disease. Person-time years at risk for 5 individuals in a hypothetical cohort study between In the above example the incidence rate for disease X is calculated as: Note that for most rare diseases, risks and rates are numerically similar because the number at risk will approximately equal the total population at all times.
Issues in defining the population at risk For any measure of disease frequency, precise definition of the denominator is essential for accuracy and clarity  The population at risk denominator should include all persons at risk of developing the outcome under investigation. Therefore, individuals who currently have the disease under study or who are immune e. However, this is not always possible in practice  Note that when individuals not at risk of the disease are included in the denominator population at risk the resultant measure of disease frequency will underestimate the true incidence of disease in the population under investigation 5.
The relationship between prevalence and incidence The proportion of the population that has a disease at a point in time prevalence and the rate of occurrence of new disease during a period of time incidence are closely related .
It provides the logical framework for the facts that enable public health officials to identify important public health problems and to delineate their dimensions. Epidemiologic methods are used to define these health problems; to classify, identify, and elucidate their causes; and to plan and evaluate rational control measures. He offered empirical insights into environmental and behavioral factors that might be associated with certain kinds of disease.
Measures of disease frequency and disease burden
Although doctors and others engaged in the healing arts did not clearly understand the concept of contagion until several hundred years later, Fracastorius c. The science of epidemiology took root with empirical observations of epidemics and other causes of death. John Graunt —in London, complied the first mortality tables on England 's bills of mortality.
Statistical analyses of deaths due to childbed fever by Ignaz Semmelweiss — in Vienna in the early nineteenth century and of tuberculosis by Pierre Charles Alexandre Louis — in Paris demonstrated the power of numbers. In London, in andmeticulous, logical examination of the facts and figures about cholera epidemics by John Snow — revealed the mode of communication of this deadly epidemic disease.
Snow is regarded as the founder of modern epidemiology because of his use of such careful methods.
Until early in the twentieth century almost all epidemiology focused on communicable diseasesalthough Percivall Pott 's — observations on cancer of the scrotum in chimney sweeps and James Lind 's dietary experiment with fresh fruit to prevent scurvy were precursors of modern noncommunicable disease epidemiology and clinical trials, respectively.
The use of epidemiology in studies of coronary heart disease and cancer in large-scale trials of many new preventive and therapeutic regimens, in nationwide surveys of health status, and in evaluation of health services came to the fore in the second half of the twentieth century. In the final quarter of the twentieth century, powerful computers, information technology, and more rigorous methodological approaches transformed epidemiology and made it a mandatory feature of clinical science as well as the most fundamental basic science of public health.
Its meaning has expanded over the years, and present-day epidemiology encompasses the study of all varieties of illness and injury as they affect defined groups of people. In a committee representing the International Epidemiological Association defined epidemiology as "the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems. Distribution includes analysis of data according to the time scale over which events occur, the places where the events occur, and the categories of persons to whom they occur.
Determinants are all the physical, biological, behavioral, social, and cultural factors that influence health. Health-related states or events include diseases, causes of death, behaviors such as the use of tobacco, reactions to preventive regimens, and provision and use of health services.
Specified populations are those with identifiable characteristics such as known numbers and age groups. The ultimate aim and purpose of epidemiology—to promote, protect, and restore good health—is manifested in the "application of this study to control health problems. One of the great intellectual challenges of epidemiology is to dissect these factors and unravel their connections in order to identify exactly what is ultimately responsible for a particular disease or health problem.
Some connections are obvious—those with vital statisticsbiostatistics, microbiology, immunology, and chemistry; with every clinical specialty from pediatrics to geriatrics and palliative care, and from family practice to hematology and neurosurgery. Other obvious connections are to the social and behavioral sciences, and, less obviously, to animal husbandrywildlife biology, agricultural science, physics, atmospheric sciences, oceanography, engineering, town planning, education, law enforcement, communications technology, and the media.
Epidemiology may be the most ecumenical of all the sciences. Probably no other branch of biomedical science has so many connections to such a wide range of other human activities.
For these comparisons to be valid, it is necessary to convert raw numbers into rates. A rate is a fraction—the upper part the numerator is the number of people affected by the problem, event, or condition of interest; the lower part the denominator is the number of persons in the population who are at risk of experiencing the problem, event, or condition.
Because the events normally continue over a long period, often indefinitely, rates are expressed in relation to a specified time. Since fractions are awkward to deal with, there is commonly a multiplier, and the rate, as shown in the following formula, is expressed in terms of so many per thousand, per hundred thousand, etc. In practice there are many variations in the ways rates are expressed, but the basic elements of events, population at risk, and time are common to all.
Rates have many uses. By comparing rates, epidemiologists can examine the experience of particular groups of people at specified times, in different cities, countries, or occupational groups. The observed differences are the basis for inferences about the reasons for these differences, and are used to test hypotheses about these reasons, possibly about the putative cause of a particular kind of cancer, for instance. In addition to the absolute requirement, for validity, of basing all comparisons on rates, another important use is in calculating the risks to individuals and groups of experiencing an event such as a heart attackthe occurrence of cancer, or traffic injury.
Comparisons are often rendered invalid, or relatively unreliable, by differences among the populations being compared—often because of failure to allow for various kinds of biases and confounding factors. A common problem stems from differences in the age composition of populations that are being compared. This problem is overcome by the procedure of age-adjustment.
Another problem is that there may be important qualitative differences, such as health or employment status, between groups that are being compared. The terms "incidence" and "prevalence" are often confused. Incidence refers to the number of new cases, events, or deaths, that occur in a specified time, usually one year. For example, about a decade ago the average duration of lung cancer was about six months.Difference between Incidence and Prevalence
Therapy was ineffective and almost all lung cancer cases died. From the time of diagnosis, the average survival was only about six months. So, the prevalence of lung cancer was fairly low. In contrast, diabetes has a long average duration, since it can't be cured, but it can be controlled with medications, so the average duration of diabetes is long, and the prevalence is fairly high.
If the population is initially in a "steady state," meaning that prevalence is fairly constant and incidence and outflow [cure and death] are about equalthen the relationship among these three parameters can be described mathematically as: Duration is the average time that people have the disease from diagnosis until they are either cured or die. If the frequency of disease is rare i.
Similarly, if the incidence remained constant, then developing a cure would reduce the average duration of disease, and this would also reduce the prevalence of disease. In the late s anti-retroviral therapy was introduced and greatly improved the survival of people with HIV. However, they weren't cured of their disease, meaning that the average duration of disease increased.