Observational Studies: A Quick read 12 min

The simplest definition of clinical trial would be “Clinical research refers to studies in which people participate as patients or healthy volunteers”. There are two main types of trials or studies - Interventional and Observational. 

Observational studies: Cohot Studies, Case control studies and Cross section studies

Cohort studies: Let’s understand the cohort first. It is a group of people who share a defining characteristic such as birth in a particular year, demography etc. Cohort studies can be divided into retrospective or prospective.

Prospective Cohort studies (observation from present to future): "People are recruited into cohort studies regardless of their exposure or outcome status. The exposed and unexposed group is followed up until the subject develops an outcome/disease of interest. For example, one could identify smokers (exposed group) and non-smokers (unexposed group) at baseline and compare their subsequent incidence of developing heart disease."

Retrospective cohort studies. (observation from present to past): “It is also called a historic cohort study. “In retrospective studies, the research is initiated after both the exposure and the outcome (e.g., disease) have already occurred. Therefore, investigators usually create two groups of people who are known to have been either exposed or not exposed to the factor of interest during a specific time frame, and then they compare the two groups with regard to their disease status. For example, A group of 100 people with AIDS might be asked about their lifestyle choices and medical history in order to study the origins of the disease. A Second group of 100 people without AIDS are also studied and the two groups are compared.”

Case control Studies: Case-control studies are retrospective in nature. The investigator has to enroll the two groups at the start: One group with disease and another group without disease. The investigator needs to look back in time to learn which subjects in each group had the exposure(s), comparing the frequency of the exposure in the case group (group with disease) to the control group.

Confusion?? Difference between case control studies and retrospective cohort studies: In retrospective cohort studies, exposure factor is known and investigatory tries to compare the risk of developing a disease to these known exposure factors. Where as in case control studies, investigator determines the possible exposure factors after a known disease incidence. Both the relative risk and odds ratio are relevant in retrospective cohort studies, but only the odds ratio can be used in case-control studies. 

Lets learn about basics of two statistics terms, 1) Relative risk: “Relative risk is a ratio of the probability of an event occurring in the exposed group versus the probability of the event occurring in the non-exposed group. An example is a relative risk of developing lung cancer (event) in smokers (exposed group) versus non-smokers (non-exposed group). “

Quite interesting to know, Why it cannot used in case control studies: In a case-control study, we cannot measure incidence, because the study starts with diseased people and non-diseased people, so relative risk can not be calculated.

2) Odd ratio: “An odds ratio (OR) is a measure of association between an exposure and an outcome. The OR represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure”You can refer the below link to learn more about odd ration and relative risk .https://www.theanalysisfactor.com/the-difference-between-relative-risk-and-odds-ratios/

Major advantage and disadvantage of cohort studies and case control studies: “Cohort studies work well for rare exposures–you can specifically select people exposed to a certain factor. But this design does not work for rare diseases–you would then need a large study group to find sufficient disease cases. Case-control studies are relatively simple to conduct. They do not require a long follow-up period (as the disease has already developed), and are hence much cheaper. This design is especially useful for rare diseases (as you select the cases yourself), but not for rare causes (as you will probably not find these in sufficient number in your study). It is also very suitable for diseases with a long latent period, such as cancer.” The case control studies are more prone to bias such as recall bias.

cross sectional studies: It is also known as transverse study, or prevalence study “The investigator measures the outcome and the exposures in the study participants at the same time. Cross-sectional studies are used both descriptively and analytically. descriptive cross sectional study simply characterize the prevalence of a health outcome in a specified population. In analytical cross-sectional studies, data on the prevalence of both exposure and a health outcome are obtained for the purpose of comparing health outcome differences between exposed and unexposed”

https://sph.unc.edu/files/2015/07/nciph_ERIC8.pdf https://canadian-nurse.com/en/articles/issues/2014/april-2014/terminology-101-retrospective-cohort-study-design https://www.ncbi.nlm.nih.gov/books/NBK430824/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2938757/ https://www.theanalysisfactor.com/cohort-and-case-control-studies-pros-and-cons/