1.1 Designing a Controlled Experiment

When scientists set up experiments they often attempt to determine how a given variable affects another variable. This requires the experiment to be designed in such a way that when the experimenter changes one variable, the effects of this change on a second variable can be measured. If any other variable that could affect the second variable were changed, the experimenter would have no way of knowing which variable was responsible for the results. For this reason, scientists always attempt to conduct controlled experiments. This is done by choosing only one variable to manipulate in an experiment, observing its effect on a second variable, and holding all other variables in the experiment constant.

Suppose you wanted to test how changing the mass of a pendulum affects the time it takes a pendulum to swing back and forth (also known as its period). You must keep all other variables constant. You must make sure the length of the pendulum string does not change. You must make sure that the distance that the pendulum is pulled back (also known as the amplitude) does not change. The length of the pendulum and the amplitude are variables that must be held constant in order to run a controlled experiment. The only thing that you would deliberately change would be the mass of the pendulum. This would then be considered the independent variable, because you will decide how much mass to put on the pendulum for each experimental trial. There are three possible outcomes to this experiment. 1. If the mass is increased, the period will increase. 2. If the mass is increased, the period will decrease. 3. If the mass is increased, the period will remain unchanged. Since you are testing the effect of changing the mass on the period, and since the period may depend on the value of the mass, the period is called the dependent variable.

In review, there are only two variables that are allowed to change in a well-designed experiment. The variable manipulated by the experimenter (mass in this example) is called the independent variable. The dependent variable (period in this case) is the one that responds to or depends on the variable that was manipulated. Any other variable that might affect the value of the dependent value must be held constant. We might call these variables controlled variables. When an experiment is conducted with one (and only one) independent variable and one (and only one) dependent variable while holding all other variables constant, it is a controlled experiment.