Cohort Studies

• They differ from clinical trials, in that no intervention, treatment, or exposure is administered to the participants. The factors of interest to researchers already exist in the study group under investigation.

• Study participants are observed over a period of time. The incidence of disease in the exposed group is compared with the incidence of disease in the unexposed group.

• Because of the observational nature of cohort studies they can only find correlation between a risk factor and disease rather than the cause. 

Cohort studies are useful if:

• There is a persuasive hypothesis linking an exposure to an outcome.

• The time between exposure and outcome is not too long (adding to the study costs and increasing the risk of participant attrition).

• The outcome is not too rare

An example using Lemons

1. Question: Does regular lemon intake affect health?

2. Design: Track a large, diverse group over time.

3. Participants: A cohort of healthy volunteers (no pre-existing conditions) with similar characteristics apart from some consume lemons others do not.

4. Baseline: Assess demographics, health, diet (including lemon intake), lifestyle, and biomarkers.

5. Follow-up: Track health outcomes (e.g., heart disease, infections) over years. Regularly monitor diet.

6. Analysis: Compare health between lemon consumption groups, accounting for other factors.

7. Goal: Identify potential links between lemons and health.

8. Challenges: Self-reported diet, confounding factors, participant retention.

9. Ethics: Informed consent, data confidentiality, clear communication of study limitations.

This prospective cohort study investigates the potential health effects of regular lemon consumption.

Retrospective Cohort Study Summary:

Title: "Association between Smoking and Lung Cancer: A Retrospective Cohort Study"

Objective: This study aimed to investigate the association between smoking and the development of lung cancer by conducting a retrospective cohort analysis.

Methods: The study utilized medical records from a large hospital database and identified a cohort of 2,000 individuals who were diagnosed with lung cancer. Detailed information on smoking habits, including duration and intensity, was collected through retrospective self-reporting. A control group of 2,000 individuals without lung cancer was randomly selected from the same database. Both groups were followed up for a period of 10 years, and the incidence of lung cancer was compared between smokers and non-smokers.

Conclusion: This retrospective cohort study provides robust evidence supporting the link between smoking and lung cancer. The findings highlight the importance of smoking cessation programs and public health interventions to reduce the burden of lung cancer.

Prospective Cohort Study Summary:

Title: "Effect of Physical Activity on Cardiovascular Health: A Prospective Cohort Study"

Objective: This prospective cohort study aimed to investigate the impact of physical activity on cardiovascular health outcomes, including the incidence of heart disease and stroke.

Methods: A cohort of 5,000 middle-aged adults without a history of cardiovascular disease was recruited from the general population. Baseline data, including demographic information, lifestyle factors, and medical history, were collected through structured interviews and questionnaires. Participants' physical activity levels were assessed using accelerometers, which measured the duration and intensity of activities over a seven-day period. The cohort was followed up for 10 years, during which cardiovascular events were documented and verified through medical records and regular check-ups.

Conclusion: This prospective cohort study provides compelling evidence supporting the beneficial effects of physical activity on cardiovascular health. The findings emphasize the importance of regular exercise as a preventive measure against heart disease and stroke. Public health initiatives should promote physical activity and encourage individuals to adopt active lifestyles for improved cardiovascular well-being.

The stages of a Cohort Study

Conducting a cohort study typically involves the following stages:

1. Study Design: Define the research question and determine the study objectives and hypotheses. Choose a prospective or retrospective cohort study design based on the availability of data and feasibility.

2. Population Selection: Identify the target population of interest and define the inclusion and exclusion criteria for participant enrollment

3. Exposure and Outcome Assessment: Determine the exposure(s) and outcome(s) of interest. Collect relevant data on exposure and outcome variables using appropriate measurement tools or data sources, such as surveys, medical records, or registries.

4. Baseline Data Collection: Collect baseline data on participant characteristics, including demographics, medical history, lifestyle factors, and other relevant variables. Ensure standardized data collection methods and quality control procedures.

5. Follow-up: Establish a follow-up period during which participants will be monitored for the occurrence of the outcome(s) of interest. Implement strategies to minimize loss to follow-up and collect data at regular intervals.

6. Data Analysis: Analyze the collected data using appropriate statistical methods, such as incidence rates, relative risks, hazard ratios, or regression models. Account for potential confounding factors and adjust the analysis accordingly.

7. Results Interpretation: Interpret the findings in light of the research question, considering the statistical significance, effect sizes, and potential biases. Analyze and report subgroup analyses, if applicable.

8. Discussion and Conclusion: Discuss the implications of the results, including strengths and limitations of the study, potential biases, and the generalizability of the findings. Compare the results with existing literature and draw conclusions based on the evidence.

9. Reporting: Prepare a comprehensive report or manuscript adhering to the reporting guidelines, such as STROBE (Strengthening the Reporting of Observational Studies in Epidemiology), ensuring transparent and accurate reporting of the study methodology, results, and conclusions.

It's important to note that cohort studies can be conducted in various settings, such as community-based cohorts, occupational cohorts, or disease-specific cohorts. The specific stages and considerations may vary based on the study objectives, available data, and field of study.

What does a strong Cohort Study look like?

• Loss of participants to follow up are stated and explanations provided.

• The control group is clearly described, including the selection methodology, whether they were from the same sample population, whether randomised or matched to minimise bias and confounding.

• It is clearly stated whether the study was blinded or not, i.e. whether the investigators were aware of how the subject and control groups were allocated.

• The methodology was rigorously adhered to.

• Involves the use of valid measurements (recognised by peers) as well as appropriate statistical tests.

• The conclusions are logically drawn from the results – the study demonstrates what it says it has demonstrated.

• Includes a clear description of the data, including accessibility and availability.

What are the pitfalls to look for?

• Confounding factors within the sample groups may be difficult to identify and control for, thus influencing the results.

• Participants may move between exposure/non-exposure categories or not properly comply with methodology requirements.

• Being in the study may influence participants’ behaviour.

• Too many participants may drop out, thus rendering the results invalid.

What are the advantages and disadvantages to consider when using a Cohort Study?

Advantages

• The only observational study design that directly investigates risk of disease and the factors contributing to it.

• Ethically safe.

• Multiple outcomes can be measured.

• They are good for rare types of exposures, e.g. an exposure to a chemical spill in a factory.

Disadvantages

• Not appropriate for rare diseases or those that take a long time to develop e.g. mesothelioma.

• Not appropriate for studying multiple exposures.

• Can be costly and time consuming.

Statistical Analysis

Risk Ratio (RR) - Uncommon (you see it more than rare pathology)

• Thinks in terms of actual probabilities of the outcome.

• Compares the risk (probability) of the outcome in an exposed group to the risk in an unexposed group.

• Imagine 100 people each in a smoking and non-smoking group. RR tells you how many times more likely someone in the smoking group is to develop the outcome compared to someone in the non-smoking group.

• An RR of 2 means someone in the exposed group is twice as likely to develop the outcome as someone in the unexposed group.

Real World Examples

Bell, A.F., Rubin, L.H., Davis, J.M., Golding, J., Adejumo, O.A. & Carter, C.S. (2018). The birth experience and subsequent maternal caregiving attitudes and behavior: A birth cohort study. Archives of Women’s Mental Health.

Dykxhoorn, J., Hatcher, S., Roy-Gagnon, M.H., & Colman, I. (2017). Early life predictors of adolescent suicidal thoughts and adverse outcomes in two population-based cohort studies. PLoS ONE, 12(8).

Feeley, N., Hayton, B., Gold, I. & Zelkowitz, P. (2017). A comparative prospective cohort study of women following childbirth: Mothers of low birthweight infants at risk for elevated PTSD symptoms. Journal of Psychosomatic Research, 101, 24–30.

Forman, J.P., Stampfer, M.J. & Curhan, G.C. (2009). Diet and lifestyle risk factors associated with incident hypertension in women. JAMA: Journal of the American Medical Association, 302(4), 401–411.

Suarez, E. (2002). Prognosis and outcome of first-episode psychoses in Hawai’i: Results of the 15-year follow-up of the Honolulu cohort of the WHO international study of schizophrenia. ProQuest Information & Learning, Dissertation Abstracts International: Section B: The Sciences and Engineering, 63(3-B), 1577.

Young, J.T., Heffernan, E., Borschmann, R., Ogloff, J.R.P., Spittal, M.J., Kouyoumdjian, F.G., Preen, D.B., Butler, A., Brophy, L., Crilly, J. & Kinner, S.A. (2018). Dual diagnosis of mental illness and substance use disorder and injury in adults recently released from prison: a prospective cohort study. The Lancet. Public Health, 3(5), e237–e248.