Incidence (epidemiology) - Wikipedia
b) Incidence c) Calculation of person-time at risk d) Issues in defining the population at risk e) The relationships between incidence and prevalence f) Commonly. Prevalence, incidence and duration of a condition or illness in a steady-state population are interrelated in such a way that two of these quantities may be used. Relationship between incidence and prevalence. changethru.info WebMaterial/ShowPic/ In table 1 the properties of all.
Raindrops might represent incidence or the rate at which new cases of a disease are being added to the population, thus becoming prevalent cases. Water also flows out of the reservoir, analogous to removal of prevalent cases by virtue of either dying or being cured of the disease. Imagine that incidence rainfall and the rate of cure or death are initially equal; if so, the height of water in the reservoir will remain constant.
If outflow from the reservoir rates of cure or death among prevalent cases remains constant and rainfall incidence of new disease increases, then the height of water in the reservoir will rise. Conversely, if incidence rainfall declines, then the water level will fall. If we start from steady state again, and the rate of rainfall remains constant, but the outflow rate of cure or rate of death increases, then the height of the water prevalence will fall.
Conversely, if incidence is held constant, but outflow falls e. Calculating Average Duration of Disease This relationship can also be used to calculate the average duration of disease under steady state circumstances.
Incidence and Prevalence | AREP
Individuals with lung cancer survived an average of 6 months from the time of diagnosis to death. If the prevalence of a disease has been more or less constant for the past ten years i. It would increase the prevalence. It would decrease the prevalence. It would have no effect on prevalence. The effect cannot be predicted from the information given. It would decrease prevalence of the disease. It would have no effect on the prevalence of disease.
The effect on prevalence cannot be predicted from the information given. True incidence is often difficult to get since access to earlier data that might contain an indication of the condition may not be available. Researchers must define a period of time in which to identify new cases.
This period of time must be long enough to capture the condition, especially in the case of a rare diseases or diseases with a low rate of diagnosis. The disease may still be common but the diagnosis may not be made or occur in administrative data unless there is a comorbid or aggravated condition. Appropriate definition of incidence will be sensitive to the condition being studied. Prevalent Cases Point prevalence - When calculating point prevalence, several years of data can still be used to identify the individuals having a condition.
In this case a single point in time would be used to identify all of those individuals in the cohort that showed the condition at that time. There are still issues around conditions that are recurrent or of a finite length i.
- Incidence and Prevalence
- Measures of disease frequency and disease burden
- Relationship Among Prevalence, Incidence Rate, and Average Duration of Disease
Period prevalence - Along with the issues of 'What' and 'When,' researchers must also answer ' Where '. This is important when using period prevalence since a single individual may have multiple ages and residence locations - even be lost to follow-up. The time period must also be determined. As with incidence, a long enough period must be used to identify the condition, but not long enough to exaggerate the actual prevalence in the population.
A period that is too long will be more affected by mortality and other loss to follow-up issues. Most of the studies involving the occurrence of a disease at MCHP use a measure of period prevalence usually looking over two or three years of data Robinson et al, The primary issues that need to be resolved here are related to changes over time - e.
MCHP researchers have taken several approaches to this problem. The two most common methods for identifying the residence location are: The age and location of residence at the first or incident case in the study.
Prevalence, incidence and duration.
The most frequent residence location, usually RHA or other large area and age that matches the population denominator in the rates measure. The age in this case is generally set as the age during the central year of the study. Issues related to age become especially problematic when there are limits on the data with regard to age. Which age is chosen to determine the cut off - age at start of study, age at incident, or age must be within the given range for the whole period of the study.
Calculation of Prevalence or Incidence When calculating prevalence, researchers must also deal with the denominator issue - who is in the population? This includes questions of risk - is the measure directed at the whole population or only those at risk?
In the case of period prevalence, the denominator may have changed over time due to loss to follow-up or migration. Typically three years of data have been used to identify conditions within the MCHP data, but other time periods can also be used.
Researchers and programmers may find point or one year prevalence easer to calculate than longer periods.