a research design where investigators observe and collect data on participants without actively assigning them to treatment or control groups. These studies are used when randomization would be impossible, impractical, or unethical, and are especially helpful for generating hypotheses that can later be tested in randomized controlled trials.
Example
Researchers follow a group of cancer patients who independently chose a new therapy and compare their survival outcomes to a group receiving standard care, without assigning or controlling who receives which therapy. This is an observational study because no randomization or assignment occurred.