In which method of research is an independent variable manipulated with an intent to observe concomitant changes in the dependent variable?

Manipulating an independent variable to observe resulting changes in the dependent variable defines the experimental method.

Manipulation plus control of other conditions lets the experiment test cause and effect.

The ex post facto method studies effects after the fact, without manipulating the independent variable.

The case study method explores one unit in depth but does not manipulate variables.

The descriptive survey method describes conditions as they are, without any manipulation.

Random assignment of participants strengthens the experiment against extraneous influences.