SBI4U Date:
Designing a Controlled Experiment

Step 1: Defining the Problem
Every scientific investigation begins with the question that the scientist wants to answer. The questions addressed by scientific inquiry are based on observations or on information gained through previous research, or on a combination of both. Just because a question can be answered doesn’t mean that it can be answered scientifically.

Step 2: Defining the Dependent and Independent Variable

Dependent Variable (DV) | Variable the experimenter measures, after making changes to the IV that are assumed to affect the DV. | Independent Variable (IV) | Variable the experimenter manipulates (i.e. changes) – assumed to have a direct effect on the dependent variable. |

Note: In many cases, the investigator does not manipulate the independent variable directly. He/She collects data and uses the data to evaluate the hypothesis, rather than doing a direct experiment. For example, the hypothesis that more crimes are committed during a full moon can be tested scientifically. The number of crimes committed is the dependent variable and can be measured from police reports. The phase of the moon is the independent variable. The investigator cannot deliberately change the phase of the moon, but can collect data during any phase he/she chooses.
Why is the scientist limited to one independent variable per experiment?
As the aim of an experiment is to see what happens when one thing is changed and how it affects something else. If you change two variables, then you don't know which variable is responsible for the change e.g. You are measuring the time it takes for something to fall. If you change both the height and the shape of the object, the time is lengthened. You don't know if the lengethening of time is a result of the change in object shape or height.

Practice:
Identify the