Table 1.
Epidemiological study designs and definitions1, and examples of epidemiological studies using metabolomics data by study design
| Study design | Definition | Examples of metabolomics studies | |
|---|---|---|---|
| Experimental | Randomized controlled trial (RCT) | A trial, or epidemiologic experiment, on humans in which the investigator controls who is exposed and who is not exposed (as well as the dose, frequency, and duration of exposure) through random allocation of study participants to exposed and non-exposed (control) groups by chance (i.e., randomization) | (Derkach et al. 2017) (Baldassarre et al. 2018) |
| Observational | Prospective cohort study | A study in which a specified group of disease-free participants, some of whom are exposed to the factor(s) of interest and some of whom are not exposed to the same factor(s) (i.e., comparison group), are followed for incidence of disease or death over a designated period of time | (Zheng et al. 2013 b) (Wurtz et al. 2015) |
| Case-control study | A study in which cases, those who have developed a given disease, and controls, those who have not developed the disease, are compared with respect to their history of past exposures of interest. The controls should be representative, with respect to the exposure(s) of interest, of the source population that gave rise to the cases | (Mathe et al. 2014) (Molins et al. 2015) | |
| Case-cohort study | A nested study in which the source population consists of cohort participants who were disease-free at baseline. Controls are randomly selected from the source population at the start of follow-up (i.e., sub-cohort). Cases are new cases of the disease of interest that developed in the source population over the designated period of follow-up | (Welsh et al. 2018) (Floegel et al. 2018) | |
| Nested case-control study | A case-control study nested within a cohort study in which the source population consists of cohort participants who were disease-free at baseline. Controls are randomly selected from those still at risk in the source population at the time a case occurs using density-based sampling. Cases are new cases of the disease of interest that developed over a designated period of follow-up | (Stepien et al. 2016) (Moore et al. 2018) | |
| Cross-sectional study | A study of the distribution of an exposure and/or a disease in a well-defined population at a single point in time, giving a “snapshot” of the population | (Holmes et al. 2008) (Carayol et al. 2017) |
1
Definitions are adapted from Modern Epidemiology, Third Edition by Rothman et al.