What type of experiment allows only one independent variable to be tested at a time?

A controlled experiment is designed specifically to test the effect of a single independent variable while keeping all other variables constant. This methodology allows researchers to isolate the relationship between the independent and dependent variables, making it easier to determine cause-and-effect relationships. By controlling all other factors, any changes in the dependent variable can be directly attributed to the manipulation of the independent variable.

In contrast, field studies and observational studies typically involve examining multiple variables in a natural setting without manipulating them, which may lead to confounding factors. Descriptive experiments focus on detailing characteristics or behaviors without demonstrating relationships or causation. Therefore, the controlled experiment stands out as the only type that effectively allows for the testing of one independent variable in a rigorous and systematic manner.