A manipulated variable is a facet of an experiment that researchers or scientists change in order to see how the outcome of the experiment changes. The manipulated variable is also called the independent variable, and it contrasts with the controlled variables, which remain constant throughout the experiment. It is important that experiments only test one manipulated variable at a time and that all other facets of the experiment remain constant.
A good example of a manipulated variable is the amount of baking soda that is added to a container of vinegar in an acid-base reaction. In such an experiment, the researcher must add increasing amounts of baking soda to the vinegar and then record the results of the reaction. While the amount of vinegar, the size of the jar, the temperature of the vinegar and numerous other aspects of the experiment are kept constant, the amount of baking soda that the investigator adds changes.
The principle of keeping experimental protocols the same while testing a single variable is at the heart of modern scientific inquiry. If researchers changed more than one aspect of an experiment at a time, they could not be sure of what caused the change in the results of the experiment.