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Cohort and Population

The words cohort and population can sometimes be used interchangeably, but their precise meanings differ.

In common practice, both of these words are used to describe a group of patients.

Examples:

The mean age of the study cohort was 55.8 years.

The mean age of the study population was 55.8 years.

However, the words have distinct meanings that make one or the other more suitable in certain contexts.

A cohort is “a group of individuals who share a common exposure, experience, or characteristic, or a group of individuals followed up or traced over time.”1 A cohort study examines a defined group of people (the cohort) over time to determine whether different characteristics or interventions are associated with different outcomes.2

Example:

There were two dose cohorts (1.5 mg/kg and 2.1 mg/kg). The drug was well tolerated in both cohorts.

The word population has a broader sense: “the total of individuals occupying an area or making up a whole”3 or “any finite or infinite collection of individuals from which a sample is drawn for a study to obtain estimates to approximate the values that would be obtained if the entire population were sampled.”1

Examples:

“The prevalence of chronic pain in cancer survivors is double that of the general U.S. population.”4

“Cancer incidence during childhood ... is approximately 10% higher in males than in females (18.2 vs 16.4 per 100,000 population).”5

No studies have examined this trend in the adolescent and young adult (AYA) population. We therefore retrospectively evaluated data for AYA patients seen at our institution from January 2004 to December 2019.

The more precise meanings of these two words are illustrated in the term population-based cohort study. From a population in a large public health database, a cohort of individuals with specific characteristics can be selected.

References
  1. Glossary of statistical terms. AMA Manual of Style, 11th edition. https://www.amamanualofstyle.com/view/10.1093/jama/9780190246556.001.0001/med-9780190246556-chapter-19-div1-6.
  2. Cohort studies. AMA Manual of Style, 11th edition. https://www.amamanualofstyle.com/view/10.1093/jama/9780190246556.001.0001/med-9780190246556-chapter-19-div2-14.
  3. Population. Merriam-Webster. https://www.merriam-webster.com/dictionary/population.
  4. Jones KF et al. Exploring factors associated with long-term opioid therapy in cancer survivors: An integrative review. J Pain Symptom Manage. 2020; in press. https://doi.org/10.1016/j.jpainsymman.2020.08.015.
  5. Siegel RL et al. Cancer statistics, 2020. CA Cancer J Clin. 2020;70:7-30. https://doi.org/10.3322/caac.21590.
--Sunita Patterson
Image Credit: © Can Stock Photo / winterling
Copyright © 2020, All rights reserved. Research Medical Library, Editing Services.


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