Prevalence Ratio in Cross-Sectional Study

Lead Author(s): Jeff Martin, MD

Definition of Prevalence Ratio

The ratio of the proportion of the persons with disease over the proportion with the exposure.

Prevalence of Disease in Exposed and Unexposed

In the 2x2 table below the proportion of the persons with disease is calculated in the group with the exposure and separately in the group without the exposure.

0311_2x2_pr.JPG Since this is a cross-sectional study, both are prevalence.

To compare these two prevalences, we can form a ratio with one over the other to get a prevalence ratio of disease in the exposed and unexposed.

Meaning of Prevalence Ratio

Strength of Association

How much greater or less than 1.0 is a measure of the strength of the association between the exposure and the disease.

This kind of measure of association is known as a ratio measure.

Ratio Based on Probabilities

In the above table: PR is ratio of two probabilities

The proportion with prevalent disease among those exposed is the probability of prevalent disease among the exposed, and similarly for the unexposed. We are making this point to distinguish a ratio based on probabilities from a ratio based on odds.

Study Reporting Prevalence Ratios

An example of the prevalence ratio can be found in Ross:

"Overall, HSV2 prevalences at follow-up were 11.9% in male and 21.1% in female participants, with adjusted prevalence ratios of: This is a publication from a community-randomized trial in Tanzania.

The main biological outcomes were the incidence of HIV infections during the trial and the prevalence of HSV2 infection at the end of the trial in the: HSV2 was measured only once, cross-sectionally, at the end of the trial and


Ross, D. A., Changalucha, J., Obasi, A. I., Todd, J., Plummer, M. L., Cleophas-Mazige, B., et al. (2007). Biological and behavioural impact of an adolescent sexual health intervention in Tanzania: a community-randomized trial. Aids, 21(14), 1943-1955.