Prevalence is **the proportion of a population who have a specific characteristic in a given time period**.

Prevalence refers to the total number of individuals in a population who have a disease or health condition at a specific period of time, usually expressed as a percentage of the population.

In science, prevalence describes a proportion (typically expressed as a percentage). For example, the prevalence of obesity among American adults in 2001 was estimated by the U. S. Centers for Disease Control (CDC) at approximately 20.9%.

Prevalence is a measure of disease that allows us to determine a person's likelihood of having a disease. Therefore, the number of prevalent cases is the total number of cases of disease existing in a population.

Prevalence refers to proportion of persons who have a condition at or during a particular time period, whereas incidence refers to the proportion or rate of persons who develop a condition during a particular time period.

Take-home point

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.

For example, in a study of incident cases of diabetes with a one year time period, only those who developed diabetes over the course of the one year study period are considered incident cases. (denominator.) The total population at risk denominator includes the prevalent cases.

Prevalence in statistics is defined as the proportion of a population which has a particular characteristic during a specified time period. It is often reported as a percentage, or as the number of cases per 10,000 / 100,000.

The prevalence is often useful as it reflects the burden of a disease in a certain population. This is not limited to burden in terms of monetary costs; it also reflects burden in terms of life expectancy, morbidity, quality of life, or other indicators.

prevalence, in epidemiology, the proportion of a population with a disease or a particular condition at a specific point in time (point prevalence) or over a specified period of time (period prevalence).

So, it can just be thought of as a wide "point in time". Example: During 1980 the Framingham Het Study examined 2,477 subjects for cataracts and found that 310 had them. So, the prevalence was 310/2,477 = 0.125. This can conveniently be expressed as 12.5 per 100 or 12.5% (per cent means 'per hundred').

Using Ratios to Compare 2 Populations

- Risk ratio (relative risk; e.g., incidence density ratio [IDR]) = Risk(Group A [Exposed])/Risk(Group B [Unexposed]) ...
- Odds ratio (OR) ...
- Rate and prevalence ratios = incidence rate or prevalence rate in the first population/ the respective rate in the second population.

Definition of Prevalence

Prevalence is determined by the size of the population and the number of outcome-positive cases. Prevalence is sometimes referred as the prevalence rate, but prevalence is actually a proportion. It can never be less than zero or greater than one.

The prevalence reflects the number of existing cases of a disease. In contrast to the prevalence, the incidence reflects the number of new cases of disease and can be reported as a risk or as an incidence rate. Prevalence and incidence are used for different purposes and to answer different research questions.

Prevalence is the proportion of a population that has a condition at a specific time, but the prevalence will be influenced by both the rate at which new cases are occurring (incidence) and the average duration of the disease.

For example, incidence is more useful than prevalence in understanding disease aetiology; this is primarily because prevalence is scaled by the average life expectancy of a disease, whilst incidence is not.

The prevalence of these forms of malnutrition is calculated by measuring the presence of malnutrition in a sample of the population selected randomly, then dividing the number of people with that form of malnutrition by the number of people in whom it was measured. Prevalence is often expressed as a percentage.

If the frequency of disease is rare (i.e., <10% of the population has it), then the relationship can be expressed as follow:

- Prevalence = (Incidence Rate) x (Average Duration of Disease)
- Average Duration = (Prevalence) / (Incidence)

Prevalence dependence on the “inflow” and “outflow” of disease according to this formula Prevalence ≈ (incidence rate) × (average duration of illness). Odds: Both prevalence and incidence proportions may be addressed in terms of odds.

For example, if 80 out of 100 exposed subjects have a particular disease and 50 out of 100 non-exposed subjects have the disease, then the odds ratio (OR) is (80/20)/(50/50) = 4. However, the prevalence ratio (PR) is (80/100)/(50/100) = 1.6.

Divide the population size by one thousand. In the example, 250,000 divided by 1,000 equals 250, which is called the quotient, the result of division. Divide the number of occurrences by the previous quotient. In the example, 10,000 divided by 250 equals 40.

However, prevalence is not a useful measure for establishing the determinants of disease in a population. Prevalence measures the frequency of existing cases of disease in a population.

The prevalence can be calculated in cross-sectional studies as a measure of frequency (figure). The prevalence describes how frequently a specific disease or a specific risk factor occurs in a population at a defined point in time. The prevalence OR can be calculated in a cross-sectional study as a measure of effect.

The Students T-test (or t-test for short) is the most commonly used test to determine if two sets of data are significantly different from each other.

A t-test is a type of inferential statistic used to determine if there is a significant difference between the means of two groups, which may be related in certain features. The t-test is one of many tests used for the purpose of hypothesis testing in statistics. Calculating a t-test requires three key data values.