These are similar to experiments, but participants aren't randomly assigned. This is often used for ethical or practical reasons—for example, you can't "assign" someone to be a smoker; you have to find people who already smoke.

Following the same group of people over a long period (years or even decades).

Comparing different groups of people at a single point in time (e.g., comparing 20-year-olds to 60-year-olds today). 5. Quasi-Experiments

An in-depth look at a single person or a small group (e.g., studying a patient with a rare brain injury).

Watching subjects in their "real world" environment without interfering (e.g., observing children on a playground).

The factor the researcher changes (e.g., dosage of a new medication).

The factor being measured (e.g., symptoms of anxiety).

This is the "gold standard" for determining cause and effect.