What is an independent variable in an experiment?

An independent variable is the factor that is manipulated or changed during an experiment to observe its effects on another variable. It is the variable that the experimenter controls, allowing them to investigate how variations in this factor influence a specific outcome, typically measured as the dependent variable. For example, if a scientist is testing the effects of different amounts of sunlight on plant growth, the amount of sunlight would be the independent variable, as it is being intentionally varied to see how it impacts the plants' growth.

The other options specify different roles of variables within an experiment. The factor that is measured relates to the dependent variable; constants refer to variables kept the same to ensure a fair test; and the observed outcome refers to the dependent variable rather than the independent variable itself. Understanding the role of the independent variable is crucial in designing experiments and interpreting their results.