Prevalence: It refers to all current cases existing at a given point in time.It's not a ratio (numerator is not part of denominator as in ratio).Prevalence is normally expressed as percentage e.g. 5% or number of cases per 10,000 or 100,000When referenced as number of cases per X, it's a proportion.Long Reference: What is Prevalence?"Prevalence" is the proportion of a population who have (or had) a specific characteristic in a given time period - in medicine, typically an illness, a condition, or a risk factor such as depression or smoking.Prevalence is calculated if one has information on the characteristics of the entire population of interest (this is rare in medicine).Prevalence is estimated if one has information on samples of the population of interest.Prevalence is generally determined by:Randomly selecting a sample (smaller group) from the entire population, with the goal being for the sample to be representative of the population.For a simple representative sample, prevalence is the number of people in the sample with the characteristic of interest, divided by the total number of people in the sample.When samples (instead of entire populations) are used to generate estimates of prevalence, statistical 'weights' may be applied to adjust the sample characteristics to match up with the target population that they were selected to represent.For example, if the randomly selected sample ends up having 53% women and 47% men in it, but the target population the sample is supposed to represent has 51% women and 49% men, then statistical weights will be applied to bring the proportions of women and men in the sample in alignment with the portions of women and men in the target population.How is Prevalence Reported?Prevalence is usually expressed as a percentage (5%, or 5 people out of 100), or as the number of cases per 10,000 or 100,000 people, depending on how common the illness or risk factor is in the population.There are several ways to measure and report prevalence, which vary according to the timeframe for the estimate:Point prevalence is the proportion of a population that has the characteristic at a specific point in time.Period prevalence is the proportion of a population that has the characteristic at any point during a given time period of interest. "Past 12 months" is a commonly used timeframe.Lifetime prevalence is the proportion of a population who, at some point in life up to the time of assessment, has ever had the characteristic.How is Prevalence Different from Incidence?"Incidence" is a measure of the number of new cases of a characteristic (such as illness or risk factor) that arise in a population over a given period (e.g., a month, a year, etc.); whereas prevalence is the proportion of a population who have (or had) a specific characteristic in a given time period, regardless of when they first acquired the characteristic.In medicine and public health, one often studies incident (new) cases to help identify causes and prevent additional cases, particularly for infectious/communicable illness.What Else Do I Need to Know about Prevalence?A variety of surveys and data systems are used to produce prevalence estimates of mental disorders. Integrating information from multiple data sources can provide useful information about the mental health of the population of interest.When examining prevalence estimates generated in a variety of surveys, it is important to take into consideration survey methodology differences that may impact the prevalence estimates produced.That is, the goals and approaches for various sources of mental health data are often different, making direct comparisons between them difficult.Some methodological differences that may affect comparisons between studies include, but are not limited to: the populations covered; the timing of data collection; sample design; mode of data collection; instruments and surveys used; operational definitions; and, estimation methods.">