PUB 540 Describe the characteristics and design of a cohort study

PUB 540 Describe the characteristics and design of a cohort study

PUB 540 Describe the characteristics and design of a cohort study

As discussed by Friis & Sellers (2020) a cohort study is an observational study that goes from cause to effect. It is a longitudinal study that follows a group of subjects over time. The exposure of a particular disease is known to be positive and how the effects of this positive exposure manifest are observed. There are two points of observation, before disease onset and then at a follow up. By doing this, a measure of effect that certain risk factors related to disease outcome. This is the best type of study for population-based studies because more measures of association can be evaluated. This is an observational study versus an experimental study because no intervention is included; the aim is to simply observe cause and effect in a certain group to determine the incidence of disease, mortality rates among this group, which then can serve as a flag for the need for more resources to prevent high numbers of untoward outcomes.

Hoepner et al., (2016) participated in a cohort study that looks at the association between bisphenol A and adiposity in the inner city. Prenatal mothers had their BPA concentrations measures (n =375), children aged 3 (n=408), and children 5 years (n=518). The children in age groups 3 and 5 had their urine spot tested. The children were followed up every 3 months and followed till they were 5 years of age, and then every 6 months until 7 years of age. The mothers were followed differently during their pregnancy. They wanted to see if there was an association between higher BPA concentration before birth and adiposity. There was a positive association that high prenatal concentrations of BPA were associated with adiposity by age 7.



Hoepner, L. A., Whyatt, R. M., Widen, E. M., Hassoun, A., Oberfeld, S. E., Mueller, N. T., Diaz, D., Calafat, A. M., Perera, F. P., & Rundle, A. G. (2016). Bisphenol A and Adiposity in an Inner-City Birth Cohort. Environmental Health Perspectives124(10), 1644–1650.

Friis, R. H., & Sellers, T. (2020). Epidemiology for public health practice (6th ed.). Jones & Bartlett Learning.

One of the most influential tools used in epidemiology is a cohort study.  According to the Center for Disease Control and Prevention reports the exposure and those who are unexposed to the disease of interest and tracks the participants to determine if they contract the disease of interest.  In the words of Friis and Sellers cohort studies are observational, and longitudinal study that begins with people who have not been exposed but have a harmful outcome.

The 2020 KH Topic I choose to discuss the “Prospective Cohort Study of Mental Health During Imprisonment”.  The aim of the scientific study was to determine how early detention or imprisonment of individuals can affect one’s mental health.  This was accomplished by screening 3079 prisoners and examining their psychiatric symptoms ( Hassan, Harty, Jarrett, Jones, King, Lathlean, Lowthian, Mills, Thornicroft, Web,  2011). (DOI: 10.1192/bjp.bp.110.080333).

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Approximately 978 prisoners exhibited psychiatric symptoms as early as the first week of incarceration.  Those who

PUB 540 Describe the characteristics and design of a cohort study
PUB 540 Describe the characteristics and design of a cohort study

remained in prison had follow-up interviews recorded at one and two. The results yielded data that depicted those psychiatric symptoms are prevalent during the first week of incarceration, but the presence tends to decline over a period. The overall study can be classified as observational because of the method used to collect data and type of data retrieved.


Hassan L, Birmingham L, Harty MA, Jarrett M, Jones P, King C, Lathlean J, Lowthian C, Mills A, Senior J, Thornicroft G, Webb R, Shaw J. (2011). Prospective cohort study of mental health during imprisonment. Br J Psychiatry 198(1), 37-42. DOI: 10.1192/bjp.bp.110.080333

Setia M. S. (2016). Methodology Series Module 1: Cohort Studies. Indian journal of dermatology61(1), 21–25.



Cohort studies do not start with an outcome but starts with exposure first. Randomization is impossible because the controls and exposures have to be chosen based on similar characteristics such as age, gender, race, and other protected discriminating factors. Setia (2016) mentions that the participants do not have an interest of outcome because we are looking at the exposures and what outcomes can result from those exposures. The main goal of the cohort study is to look for any risk factors that resulted in the exposures that may result in unhealthy outcomes.




Setia M. S. (2016). Methodology Series Module 1: Cohort Studies. Indian journal of dermatology61(1), 21–25.

Strengths of Cohort Studies

Much of the medical profession’s current knowledge of disease risk factors comes from cohort studies. In addition to showing disease progression, cohort studies also help researchers calculate the incidence rate, cumulative incidence, relative risk, and hazard ratio of health conditions.

  • Size: Large cohort studies with many participants usually give researchers more confident conclusions than small studies.
  • Timeline: Because they track the progression of diseases over time, cohort studies can also be helpful in establishing a timeline of a health condition and determining whether specific behaviors are potential contributing factors to disease.
  • Multiple measures: Often, cohort studies allow researchers to observe and track multiple outcomes from the same exposure. For example, if a cohort study is following a group of people undergoing chemotherapy, researchers can study the incidence of nausea and skin rashes in the patients. In this case, there is one exposure (chemotherapy) and multiple outcomes (nausea and skin rashes).
  • Accuracy: Another strength of cohort studies—specifically, prospective cohort studies—is that researchers might be able to measure the exposure variable, other variables, and the participants’ health outcomes with relative accuracy.
  • Consistency: Outcomes measured in a study can be done uniformly.

Retrospective cohort studies have their own benefits, namely that they can be conducted relatively quickly, easily, and cheaply than other types of research.

According to (2019) cohort study is a design where one or more samples (called cohorts) are followed prospectively and subsequent status evaluations with respect to a disease or outcome are conducted to determine which initial participants exposure characteristics (risk factors) are associated with it.

The health condition I have identified is environmental quality. (2022) discusses how the Air Quality Index (AQI) reports daily air quality from a value of 0 to 500, considers values greater than 100 to indicate unhealthy levels of air pollution. It was reported that between 2006–2008 and 2016-2018, potential exposure to unhealthy air quality (measured as the number of AQI-weighted people days) decreased 43%, from 7.603 to 4.296 billion, exceeding the HP2020 target (, 2022).

The link to the article is

As stated on (2022) between 2005–2008 and 2013-2016, exposure to secondhand smoke among children aged 3–11 years was conducted and it was discovered that there was a decrease of 27.0%, from 52.2% to 38.1%, exceeding the HP2020 target. In 2013-2016, several groups of children in specific demographic categories had the lowest rates of secondhand smoke exposure, including Asian children, those born outside the U.S., those with private health insurance, and those in families with incomes at 500% or more of the poverty threshold (, 2022).

Friis & Sellers (2021) explain that a cohort study as an observational study design makes use of careful measurement of patterns of exposure and disease in populations to draw inferences about etiology while an experiment study design would be impractical and, in some instances, unethical.


Friis, R.H. & Sellers, T.A. (2021). Epidemiology for public health practice (6th ed.). Burlington,

MA: Jones & Bartlett Learning. (2022). Environmental Quality. Retrieved from (2019). Study Design 101. Retrieved from,factors)%20are%20associated%20with%20it.