Endgames Statistical Question

Prevalence and incidence

BMJ 2010; 341 doi: https://doi.org/10.1136/bmj.c4709 (Published 01 September 2010) Cite this as: BMJ 2010;341:c4709
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London
  1. p.sedgwick{at}sgul.ac.uk

    A randomised controlled trial investigated whether screening and treating women for chlamydial infection reduced the subsequent occurrence of pelvic inflammatory disease.1 Between 2004 and 2006, vaginal swabs were provided by 2529 sexually active female students. The samples were randomly allocated to immediate testing (screening group), or storage and deferred screening after a year (control group). After screening, treatment for chlamydial infection was offered to women where necessary. All women were able to undertake independent testing for chlamydia during the follow-up period of one year.

    At baseline, 5.4% of the women in the screened group tested positive for chlamydia compared to 5.9% of the controls. Over the following twelve months, 1.3% of the screened women developed pelvic inflammatory disease compared with 1.9% of the controls.

    Which of the following statements, if any, are true?

    • a) The percentage of women with chlamydia at baseline estimated the population prevalence

    • b) Women who developed pelvic inflammatory disease during follow-up are known as incident cases

    • c) The proportion of the control group that developed pelvic inflammatory disease in the follow-up period estimated the population incidence

    • d) The incidence rate of pelvic inflammatory disease for the control group was 19 cases of pelvic inflammatory disease per 1000 woman years


    a, b, c, and d are all true.

    Prevalence and incidence quantify the extent and burden of a disease or condition in a population. Prevalence is the number of cases of a disease or condition in the population expressed as a proportion or percentage of the population at risk. Prevalence measured at a particular point in time—for example, on a certain day—is known as point prevalence, often referred to simply as prevalence. Point prevalence is useful for comparing different points in time to help determine whether an outbreak of an infectious disease is occurring. Prevalence can also be measured over a period of time, typically a year, when it is then known as period prevalence.

    Incidence is the number of new cases of a disease or condition that occur over a particular time period. Incidence can be expressed as a proportion or percentage—that is, the number of new cases divided by the size of the population at risk at the start of the study period.

    The female students in this study were a convenience sample selected from universities and further education colleges in London and represented the population of sexually active female students. Although the randomised treatment groups might not have been at baseline with respect to demography, clinical history, and disease severity, they still estimate the population. Therefore, the percentage of students at baseline with chlamydia in either group estimated the population prevalence (a is true). Although women were recruited across several years, with the swabs for the control group not screened until a year after recruitment, each woman was screened only once. When estimating the population prevalence of chlamydia, the authors of this study assumed that the epidemiology of chlamydia did not change with time.

    Women who developed pelvic inflammatory disease during the one year follow-up are called incident cases (b is true). The control group estimated the population without any intervention. Therefore, the proportion of the control group that developed pelvic inflammatory disease during the follow-up period (0.019 or 1.9%) estimated the population incidence of pelvic inflammatory disease in one year (c is true).

    All women in the trial were followed for a year and, therefore, had the same risk of developing pelvic inflammatory disease. If the length of follow-up was not standardised, it might have been difficult to estimate the population incidence. Women followed for longer periods would have had a greater opportunity to develop pelvic inflammatory disease than those followed for only short periods. This is a particular problem in cohort studies where members of the cohort are followed for different lengths of time. Therefore, incidence is often expressed as a rate.

    The incidence rate for pelvic inflammatory disease would be the number of incident cases divided by the sum of the follow-up times for women in the sample, which would give the rate at which pelvic inflammatory disease occurred per woman per period of time. Incidence rates are typically calculated per annum. Given that each woman was followed for a year, the total number of years of follow-up in this study was equal to the number of women. Therefore, the incidence rate of pelvic inflammatory disease for the control group was 0.019 cases per woman per year. As the rate per person year is small, it is more convenient to multiply by a factor of 1000 and express the incidence rate as 19 cases of pelvic inflammatory disease per 1000 woman years (d is true). Incidence rates are useful for comparing studies that have different lengths of follow-up, or when individuals within studies are followed for variable lengths of time.


    Cite this as: BMJ 2010;341:c4709


    • Competing interests: None declared.


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