# Prevalence Ratio, Odds Ratio and Relative Risk

I would like to know the differences among Prevalence Ratio, Odds Ratio and Relative Risk. Could any one please tell me?

I would like to know the differences among Prevalence Ratio, Odds Ratio and Relative Risk. Could any one please tell me?

## Popular Answers

Eduardo J Simoes· University of MissouriRisk is the probability of occurrence of a new event (say health outcome) over a period of time among those who are at risk for event occurrence (say developing/acquiring health outcome) at the beginning of the follow up period. Risk is usually expressed as a value between 0 and 1, as all probabilities. It is often estimated through follow-up or Cohort studies of two groups of subjects/individuals (with or without some characteristics/attribute – usually called exposed and unexposed groups). So, the Risk estimates the average probability of occurrence of an outcome over a specified period of time among individuals at risk for having the outcome at the beginning of the follow-up period.

Prevalence is the number or proportion of subjects/individuals with some attribute (can als be the level of attribute/characteristics) or outcome (event) in a point in time or period of time. Prevalence is usually expressed as a proportion (0 to 1 as probabilities; or as percentages). It is usually derived from cross-sectional studies or a random sample from a population of subjects/individuals in a point in time (or period of time) – no need to specify a follow up period.

Therefore, Prevalence Ratio (erroneously called Prevalence Rate Ratio- because prevalence is not a rate) indicates how large is the prevalence of an event/outcome in one group of subjects/individuals (with characteristics/attribute) relative to another group (without the characteristics/attributes).

While, the ratio of the risks (Risk Ratio or Cumulative Incidence Ratio) indicates how more or less likely one a group of individuals/subjects with attribute/characteristics (exposure) is to develop/acquire a health outcome or conditions over the follow up period relative to the other group of unexposed.

The mathematic of the Odds Ratio presented are also correct, but colleagues are freely throwing the concepts of Prevalence Odds Ratio and Risk Odds Ratio as interchangeable, while they are not for the reasons discussed above. The Exposure Odds Ratio of derived from Case-Control Studies or a Risk Odds Ratio derived from a Cohort study are only approximated estimates of RisK Ratio (or Relative Risk). The Prevalence Odds Ratio is not an approximate measure of the Risk Ratio - it is a paremter on its own that can be used in cross-sectional studies as a measure of the association between exposure/factors and the outcome of interest, without any need to mention Risk ratio.

In any circusntance in which an Odds Ratio is being estimated, it is important to know which fundamental measure is being estimated indirectly by the Odds Ratio: Risk ratio or Prevalence Ratio.

Finally, in exceptional circunstances the Prevalence may estimate another epidemiological measure the Incidence Rate (or incidence density), a true rate. In this situation the Prevalence Ratio is actually estimating the Incidence Rate Ratio.

## All Answers (8)

Deleted· National Research CouncilYin Myo AyeDeleted· National Research CouncilYin Myo AyeAlan Nyitray· Moffitt Cancer CenterYin Myo AyeEduardo J Simoes· University of MissouriRisk is the probability of occurrence of a new event (say health outcome) over a period of time among those who are at risk for event occurrence (say developing/acquiring health outcome) at the beginning of the follow up period. Risk is usually expressed as a value between 0 and 1, as all probabilities. It is often estimated through follow-up or Cohort studies of two groups of subjects/individuals (with or without some characteristics/attribute – usually called exposed and unexposed groups). So, the Risk estimates the average probability of occurrence of an outcome over a specified period of time among individuals at risk for having the outcome at the beginning of the follow-up period.

Prevalence is the number or proportion of subjects/individuals with some attribute (can als be the level of attribute/characteristics) or outcome (event) in a point in time or period of time. Prevalence is usually expressed as a proportion (0 to 1 as probabilities; or as percentages). It is usually derived from cross-sectional studies or a random sample from a population of subjects/individuals in a point in time (or period of time) – no need to specify a follow up period.

Therefore, Prevalence Ratio (erroneously called Prevalence Rate Ratio- because prevalence is not a rate) indicates how large is the prevalence of an event/outcome in one group of subjects/individuals (with characteristics/attribute) relative to another group (without the characteristics/attributes).

While, the ratio of the risks (Risk Ratio or Cumulative Incidence Ratio) indicates how more or less likely one a group of individuals/subjects with attribute/characteristics (exposure) is to develop/acquire a health outcome or conditions over the follow up period relative to the other group of unexposed.

The mathematic of the Odds Ratio presented are also correct, but colleagues are freely throwing the concepts of Prevalence Odds Ratio and Risk Odds Ratio as interchangeable, while they are not for the reasons discussed above. The Exposure Odds Ratio of derived from Case-Control Studies or a Risk Odds Ratio derived from a Cohort study are only approximated estimates of RisK Ratio (or Relative Risk). The Prevalence Odds Ratio is not an approximate measure of the Risk Ratio - it is a paremter on its own that can be used in cross-sectional studies as a measure of the association between exposure/factors and the outcome of interest, without any need to mention Risk ratio.

In any circusntance in which an Odds Ratio is being estimated, it is important to know which fundamental measure is being estimated indirectly by the Odds Ratio: Risk ratio or Prevalence Ratio.

Finally, in exceptional circunstances the Prevalence may estimate another epidemiological measure the Incidence Rate (or incidence density), a true rate. In this situation the Prevalence Ratio is actually estimating the Incidence Rate Ratio.

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