Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

When manipulating variables in an experiment, what is the best practice?

1. Manipulate multiple variables at once

2. Only manipulate the dependent variable

3. Keep only one variable manipulated at a time

4. Use random assignment of variables

The correct answer is: Keep only one variable manipulated at a time

In experimental design, the most effective approach is to keep only one variable manipulated at a time. This practice, often referred to as controlling variables, allows researchers to isolate the effects of the manipulated variable on the outcome or dependent variable. By ensuring that only one variable is changed, the experimenter can clearly determine the cause-and-effect relationship between the manipulated variable and the measured outcome. This rigorous control minimizes confusion and helps attribute any observed changes directly to the variable being tested. Manipulating multiple variables at once can lead to confounding factors, where it becomes difficult to discern which variable is responsible for any changes in the outcome. This lack of clarity can compromise the validity of the experiment. While random assignment of variables can enhance the reliability of results by reducing bias, it does not replace the importance of manipulating only one variable at a time for establishing clear causation.