TL;DR Prevalence: Number of people in a population who have a disease or other health outcome at one point in time. Prevalence includes all cases, both new and pre-existing, in the population at the specified time, whereas incidence is limited to new cases only. These measures work together to help us understand and plan for the impact of a disease or health outcome in our community.
Why do we need to study both? Facebook Twitter LinkedIn Email. Related Posts. September 1st, July 7th, May 25th, March 14th, Often these words are used interchangeably however, when it comes to surveillance and epidemiological analyses these words have both distinct meanings and purpose. To help understand the difference between incidence and prevalence, epidemiologists often refer to the familiar concept of water entering and leaving a bath. The prevalence is the number of individuals with the disease either at a specific point in time the point prevalence or over a specified time period the period prevalence.
The key concept about the prevalence is that it includes both new and existing cases of disease. If we refer to our bathtub, the prevalence is the amount of water in the bathtub at a specified point in time. As we all know, this quantity is not static and we can easily add more water, by turning on the tap, as well as lose water through evaporation or by opening the plug. Turning back to our disease model, the filled bath represents the total number of cases at a point in time which can change through evaporation or releasing the plug as individuals recover from the disease or if they unfortunately pass away.
Therefore, the prevalence of a disease in a population is influenced by a number of parameters: the number of new cases turning on the tap , the speed of recovery — which is influenced by disease severity and treatment options evaporation and the number of people that lose their lives to the disease releasing the plug.
Therefore, we often express the prevalence as a rate using the population as a denominator. Stakeholder Engagement Find out how NIMH engages a range of stakeholder organizations as part of its efforts to ensure the greatest public health impact of the research we support. Education and Awareness Use these free mental health education and outreach materials in your community and on social media to spread the word about topics like eating disorders, autism awareness, and suicide prevention.
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Press Resources Information about NIMH, research results, summaries of scientific meetings, and mental health resources. Innovation Speaker Series NIMH hosts an annual lecture series dedicated to innovation, invention, and scientific discovery. Gordon, M. Incidence proportion, risk or cumulative incidence refers to the number of new cases in your population during a specified time period. It can be calculated using the following equation:. Incidence rate incorporates time directly into the denominator and can be calculated as follows:.
Person years at risk means the total amount of time in years that each person of the study population is at risk of the disease during the period of interest. Prevalence differs from incidence proportion as prevalence includes all cases new and pre-existing cases in the population at the specified time whereas incidence is limited to new cases only.
The factor connecting prevalence and incidence statistics is the length of time the disease is present for before patients die, or are cured.
The number of people that are diagnosed with asthma every year is quite static over time. It has a genetic component, is not being affected hugely by lifestyle or other factors and the rate of cases does not particularly change. This means that the incidence of the disease stays fairly constant.
However, nowadays people do not die of asthma and the number of people with asthma in the population persists until they die of another cause. There is also not often a cure for asthma, it does not go away but it is just managed better. Therefore the number of cases of asthma keeps increasing while more people are diagnosed with the condition than those with the disease that die. The prevalence of asthma in a population is therefore increasing.
At the other end of the spectrum is a very short-lived condition with high mortality, such as Ebola. The number of cases of Ebola increased massively during the epidemic, making the incidence extremely high.
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