Scientific Literature Search Strategies – Part II: Clinical Studies

The National Institute of Health created an article on Understanding Clinical Studies in 2016 and included a useful infographic. More detail about clinical research that is not covered in this blog post can be found on

Observational studies are unable to prove cause and effect, but are useful for finding associations and sparking hypotheses, especially in instances of genetic, environmental, behavioral, and infections causes of disease.

There are five types of observational studies listed below in ascending level of scientific strength:

  1. Case Study/Case Series – Detailed documentation of one or more patients, specifically new or unusual cases to add evidence to the scientific community for the purpose of generating hypotheses to be used in future studies.
  2. Ecological Studies – Comparison of groups of people with different rates of a disease or condition to probe for associations with environmental or socioeconomic factors.
  3. Cross-Sectional Studies – Data from many people in a snapshot in time across to identify prevalence and associated factors.
  4. Case-Control Studies – Comparison of a group of people with a condition to a group of people without the conditions to identify possible causes or risk factors.
  5. Cohort Studies – Observation of a large group of people without a condition or disease. Some go on to develop the disease or condition and the data collected can find possible cases or risk factors for that disease or condition.

A well-designed clinical trial is usually necessary to establish cause and effect and can be used when trying to scientifically validate a way to prevent, detect, or treat a disease. The current topic in the news is the COVID-19 pandemic. Understanding clinical trials can help explain why it was anticipated it would take many months and likely more than a year (if possible) to bring a safe and effective vaccine to the public. A strategy to speed up the eventual availability of a COVID-19 vaccine has been to mass produce multiple potential vaccines before the conclusion of clinical trials.  This potentially allows immediate market available of successful vaccine(s), while mass produced unsuccessful vaccine candidates will be destroyed.

Clinical Trials involve four phases summarized below.

  • Phase I – Assess safety, identify side effects, and find appropriate dose (typically involves fewer than 100 people)
  • Phase II – Test efficacy, monitor side effects, and obtain data to design a larger Phase III trial (typically involves 100-300 people)
  • Phase III – Prove efficacy and monitor side effects (involves 1,000 or more people)
  • Phase IV – Market the medical approach and continue to gather data (involves 1,000s of people)

More information about Clinical Trials and how to enroll a patient can be found on the NIH website.