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Guest Lecture – Tashiyana Devanathan Study designs: Case control and Cohort Introduction/ Recap Study designs are structured approaches to address specific research questions- looks at the relationship between exposure and outcome to answer a research question. Outcomes- Health outcome of interest in the study- Disease, death, side effect, complication (stats: “dependent variables” Y axis ) “Exposures”- Measures that may be associated with the outcome - Possible “risk factors”, “causes”, “determinants” (stats: “independent variables” X axis ) Already familiar with cross-sectional which looks at exposure and outcome simultaneously, today’s focus on case-control and cohort study designs. Case-control study design You have the outcome, you are investigating exposure In order to do this: Compares a group of participants possessing a condition of interest (cases) to a similar group who do not possess that condition of interest (controls)- Those who have the outcome compared to those who don’t. The case group is chosen because they already possess the attribute of interest (disease, condition, complication)- outcome The point of a control group is to facilitate investigation whether the case group is more likely to exhibit the outcome than the controls.
The control group should have similar demographic characteristics to the case group except for the outcome being investigated in order to produce the most accurate results. More controls= more accurate results The same set of questions is asked to both groups in order to investigate exposure related to the outcome. This is a good way of looking for risk factors for a disease. Example: To investigate if smoking is associated with an increased risk of lung cancer. Cases: Patients who have been diagnosed with lung cancer (the outcome of interest). Controls: Patients who do not have lung cancer but are otherwise similar in age, gender, and other relevant factors. Exposure Assessment: Gather information on the smoking history of both the cases and controls, such as whether they have smoked, for how long, and how much. Analysis: Compare the frequency of smoking between the cases and controls. If smoking is more common among the cases than the controls, it may suggest an association between smoking and lung cancer. Draw a conclusion based on your results- is the exposure related to the outcome? Advantages of case-control studies: Good design for rare, chronic and long latency diseases Relatively inexpensive (population size and time) Allows for the examination of multiple exposures Estimate odds ratios Hospital-based studies and outbreaks Disadvantages of case-control studies: Multiple outcomes cannot be studied Recall bias Sampling bias
Cohort study designs Opposite of case-control study design- you have the exposure, you are investigating the outcome. A cohort is a group of people who share a common characteristic or experience within a defined period. In research, a cohort often refers to a group of individuals who are followed over time to study the impact of different exposures on their health outcomes. For example, a cohort could be a group of people born in the same year, people who started smoking at the same time, or people who were exposed to a particular environmental factor. In a cohort study you start with a group of people who are exposed to something (like smoking) and another group who are not exposed. You follow both groups over time to see who develops the outcome (like lung cancer). Example: Tracking smokers and non-smokers over several years to see who gets lung cancer- note how this is different to a case-control study. Key differences: Cohort Study: Follows people forward in time from exposure to outcome. Case-Control Study: Looks backward in time from outcome to exposure. Types of cohort studies:
- Prospective - taking a current group of individuals with the exposure and documenting them going forward over a period of time to find the outcomes (moving forward)
2. Retrospective - researchers look back at data from a group of people (a cohort) who were exposed to something in the past and compare their outcomes to those who were not exposed. The key here is that the events (exposures and outcomes) have already happened, and the researchers are analysing existing records or data (looking back) How is a retrospective cohort study different from a case-control study? Retrospective Cohort Study: Starts with people based on their past exposure and follows up on what outcomes they developed. The focus is on a timeline where exposure happened first, and then you check the outcomes. Case-Control Study: Starts with people who already have the outcome (cases) and those who don't (controls), then looks back to see if they were exposed to something in the past.
Risk Ratio (RR) Risk Ratio (RR) , also known as the Relative Risk, is a measure used in cohort studies to compare the risk of a specific outcome between two groups: those exposed to a certain factor and those not exposed. In cohort studies, the Risk Ratio is used to determine how much more (or less) likely the exposed group is to develop the outcome compared to the unexposed group. It provides a direct measure of the association between exposure and outcome by comparing the incidence rates between the two groups. Interpretation of RR RR= 1: The exposure has no effect on the outcome RR>1: The exposure causes the outcome RR<1: The exposure is protective of the outcome Types of Bias: Selection bias : Bias in whom you choose to participate in a study. This occurs when the participants selected for a study are not representative of the target population, leading to skewed results. Recall Bias: This happens when participants do not remember past events or exposures accurately, especially if the outcome has already occurred. Information Bias: This arises from errors in measuring or recording information about the exposure or outcome, leading to incorrect data.
Misclassification Bias: This occurs when participants are incorrectly categorized regarding their exposure or outcome status, leading to inaccurate associations in the study. It is important to note which bias may serve as a disadvantage for each study design When should you use a cohort study design?
- You want to examine the impact of exposures on future outcomes
- The exposure is rare but the outcome is common
- You want to study multiple outcomes
- You are able to follow participants over time Exercise For the scenarios below, state which study design would be best suited?
- A researcher wants to determine if exposure to a specific chemical is linked to a rare type of cancer. (case-control)
- A researcher wants to study whether a diet high in fruits and vegetables lowers the risk of developing cardiovascular disease over the next 10 years. (cohort)
- A researcher wants to evaluate whether a new vaccine prevents the onset of a particular infectious disease. (cohort)
- A researcher wants to identify if there is an association between high cholesterol levels and the risk of heart attacks among individuals who have already experienced a heart attack versus those who have not. (case-control)
- A researcher aims to study the impact of smoking on the development of chronic obstructive pulmonary disease (COPD) by tracking individuals who smoke and those who do not over several years. (cohort)
- A researcher wants to determine if a high intake of processed meats is linked to the risk of colorectal cancer by looking at dietary records and cancer diagnoses in a group of patients. (case-control)
- A researcher is interested in whether long-term exposure to air pollution is associated with an increased risk of asthma. (cohort)
