In clinical research, the study can be designed in a multitude of ways. Each design has different purpose and limitations. Below is a list of the most common types of peer-reviewed studies:
Meta-Analysis
In a meta-analysis, data from many different studies (often RCT) of the same treatment or phenomenon are combined. The aim is to statistically process the results from the various studies to find the best evidence for, e.g., the effect of a treatment.
Example: Thanapluetiwong S. et al. (2020) Vitamin D supplement on prevention of fall and fracture: A Meta-analysis of Randomized Controlled Trials. Medicine (Baltimore), 99(34).
Systematic Review (SR)
A systematic review is a structured summary and critical assessment of original research (the primary literature, eg RCT studies) based on a pre-defined specific research question e.g. in clinical research. A SR includes thorough and methodical literature search, inclusion criteria, selection procedures, data extraction and data analysis.
Example: Li G, Mbuagbaw L et al (2014) Efficacy of vitamin D supplementation in depression in adults: a systematic review. J Clin Endocrinol Metab. 99(3):757-67.
Randomized controlled trials (RCT)
RCTs are research studies used to show the impact of an intervention, e.g. a medical treatment. The participants in the study are divided randomly into two or more groups. What you want to investigate the effect of is given to one group, the "experimental group", after which you compare with the other group, the "control group", which e.g. has received placebo and thus has not been exposed to the intervention. In double-blinded RCT studies, it is unknown to researchers, experimental groups and control groups who is receiving treatment and who is receiving placebo. In single-blinded RCT studies, the researcher knows who is receiving which treatment, i.e. only the experimental group and the control group do not know it.
Example: Soininen H et al. (2017). 24-month intervention with a specific multinutrient in people with prodromal Alzheimer's disease (LipiDiDiet): a randomised, double-blind, controlled trial. Lancet Neurol. 16(12):965-975.
Cohort Study
Cohort studies are research studies based on the observation of a population, which for example are being treated with a specific medicine or suffering from a specific disease. The population is monitored for a certain period of time and compared with another population that does not receive the treatment or suffers from the disease.
Example: Schreiber PW et al. (2019) Swiss Transplant Cohort Study (STCS). Vitamin D status and risk of infections after liver transplantation in the Swiss Transplant Cohort Study. Transpl Int. 32(1):49-58.
Case-control Study
In case-control studies, a population with a specific outcome is selected, e.g. disease cases. Then you interview them about what they have been exposed to or you look at data from the people in question and compare with another population who have been exposed to the same thing but who do not have the disease.
Example: Chouët J et al. (2020) Vitamin D and Delirium in Older Adults: A Case-Control Study in Geriatric Acute Care Unit. Front Neurol. 2020 11:1034.
In the following references, you can read more about different types of study design:
Literature Reviews: Types of Clinical Study Designs. Georgia State University Library.
Meinert, C. L. (2012). Clinical trials handbook: design and conduct. John Wiley & Sons.