Twin Study

Twin studies are used to determine whether genetic or environmental factors produce a certain outcome - also known as the "nature vs. nurture" debate. Because twins have identical genetic makeup, researchers can test whether traits or conditions are inherited or a result of external factors. 

In this example, which had a study sample of 766 pairs of twins, researchers are exploring the role of genetics in the trajectory of aggressive and delinquent behavior in young people. This is also a longitudinal study, as the twins participated in 4 waves over several years, at ages 9-10, 11-13, 14-15, and 16-18. 

Isen, J., Tuvblad, C., Younan, D., Ericson, M., Raine, A., & Baker, L. A. (2022). Developmental trajectories of delinquent and aggressive behavior: Evidence for differential heritability. Child Psychiatry and Human Development, 53(2), 199–211. https://doi-org.nvcc.idm.oclc.org/10.1007/s10578-020-01119-w

Clinical Trial

A clinical trial is done to test a new medical intervention such as a medication to treat a disease or disorder or a vaccine to prevent against a disease. In a clinical trial, one group will get the treatment or intervention and one group will get a placebo. The group that gets the placebo is called the control group. This way, researchers can determine if the group that received the intervention had a significantly different outcome from the group that did not receive the intervention. 

In this example, researchers are testing cognitive behavioral therapy (CBT) in collaboration with medication as a treatment for anxiety. 

Campbell, S. L., Roy, B. P. P., Craske, M. G., Bystritsky, A., Sullivan, G., & Stein, M. B. (2016). Improving outcomes for patients with medication‐resistant anxiety: Effects of collaborative care with cognitive behavioral therapy. Depression and Anxiety, 33(12), 1099–1106. https://doi-org.nvcc.idm.oclc.org/10.1002/da.22574

You can find an extensive list of current clinical studies at ClinicalTrials.gov from the U.S. National Library of Medicine. 

Cross-sectional Study vs. Longitudinal Study

The classification of a study as cross-sectional or longitudinal has to do with the timing of when participants are observed or tested. In a cross-sectional study, participants are observed or tested at one point in time. In a longitudinal study, participants are observed or tested more than once over a period of time - for example, in a study about smoking cessation aids, a group of smokers may be surveyed at an initial encounter, then 2 months later, and then a year later, to determine whether the intervention helped them to quit smoking. Longitudinal studies can be done over many years.