We typically categorise clinical studies depending on their purpose and design. The principal study designs are observational studies, randomised controlled trials, cohort studies, case control studies, cross sectional studies, systematic reviews and meta analyses.
The goal of these studies is to observe population characteristics between groups without actively intervening. These studies are often used to explore associations between variables, like the relationship between a person's diet and their risk of developing a particular condition.
We distinguish between prospective observational studies, where data is collected on individuals who have not yet developed the outcome of interest, and retrospective observational studies, where data is collected on individuals having already developed the outcome.
These studies help provide into real-world insights on populations and groups, thereby helping identify potential risk factors or protective factors. However, observational studies cannot establish cause and effect relationships because they do not involve an intervention.
Randomised controlled trials (RCTs)
These studies randomly assign participants to an intervention group receiving the treatment being tested and a control group receiving either a placebo or standard care. To evaluate whether the intervention is effective, each group's outcome is then compared.
RCTs are often referred to as the gold standard of clinical research, because they provide a high level of evidence and they help minimise bias (thanks to the random allocation of individuals in the two groupe).
However, RCTs are expensive and time-consuming, and they may not always be feasible, e.g., for ethical reasons. If there is strong evidence that the intervention is effective and withholding the intervention from the control group is harmful, then it's unethical to randomly assign some participants to the control group. Similarly, if an intervention is known to be harmful, then it's also unethical to randomly assign participants to the intervention group.
In situations where RCTs are not applicable, alternative clinical study designs exists like observational studies.
Cohort studies focus on a group of people — who are similar in some way or who have been exposed to the same risk factor, follow cohort members over time, and then collect data on each cohort members' characteristics, behaviours, and outcomes.
The analysis of characteristics, behaviours, and outcomes of these cohorts aims to reveal patterns or trends in the data, e.g., about the natural history of a disease or the potential risk factors or protective factors.
However, cohort studies are expensive and time-consuming, and they are subject to bias if participants drop out of the study or if there are differences between the participants who remain in the study and those who do not.
Case control studies
Case control studies compare two groups of individuals: the first group has a specific condition (the cases), whereas the second group does not have that condition (the controls). These studies collect data on the potential risk factors or exposures for individuals in the two groups. And then the analysis compares the two groups to see if there are any differences.
Case-control studies are useful because they can be conducted quickly and inexpensively, and they help generate hypotheses on the potential causes of a particular condition or disease. However, case-control studies are subject to bias, e.g., if the selection process for the cases and controls differs significantly.
Cross sectional studies
Cross-sectional studies collect data on a group of individuals at a single point in time and then analyse the relationship between different variables, looking for patterns or trends in the data.
Cross-sectional studies are useful to describe the characteristics or prevalence of a particular condition or disease in a population at a given point in time — the "snapshot". They help identify potential risk factors or protective factors.
Cross-sectional studies have the advantage of being rather quick and inexpensive to conduct, however they are subject to bias, e.g, if the sample are not representative of the population of interest.
Meta-analyses and systematic reviews
A meta-analysis involves searching for and identifying systematically all the studies on a particular topic, combining the results, analysing the data, and then summarising the findings.
By pooling data from a large corpus of studies, meta-analysis aim to provide a more robust estimate of the effect of an intervention .
Meta-analyses can provide valuable insights that individual studies fail to reveal. They may also help to identify inconsistencies. However, they are only as reliable as the studies that they include, and they may be subject to bias if the selection of studies is not done properly.
Clinical studies aim to evaluate the effectiveness and safety of medical interventions like drugs or medical devices.
Clinical studies are typically categorised based on their goal and design:
- Observational studies observe and record data on a group of people without actively intervening;
- Randomized controlled trials (RCTs) randomly assign participants to either the intervention group or the control group and compare the outcomes of the two groups to determine the intervention effectiveness;
- Cohort studies follow a group of people over time and collect data on their characteristics, behaviors, and outcomes;
- Case-control studies compare a group of individuals with a specific condition to a group of individuals without the condition and look for differences between the two groups;
- Meta-analyses combine and analyze the results of multiple studies to provide a more comprehensive understanding of a particular research topic.