* Manipulation of an independent variable: The researcher must actively manipulate at least one independent variable to observe its effect on a dependent variable. This is what distinguishes experiments from other research methods like observational studies.
* Random assignment: Participants must be randomly assigned to different groups (e.g., treatment and control groups) to minimize pre-existing differences between groups that could confound the results. Random assignment helps ensure that any observed differences between groups are due to the manipulation of the independent variable, not other factors.
* Control of extraneous variables: Researchers must minimize the influence of other variables (extraneous variables) that could affect the dependent variable, besides the independent variable. This is done through various methods, such as using control groups, matching participants, or statistical control.
While other elements are important (e.g., clear hypotheses, reliable and valid measurement), control over the variables is the defining feature that distinguishes an experiment from other research designs and allows for causal inferences to be drawn.