Isn't it possible that conditions of an experiment differ in other ways, besides the independent variable?

• Holding conditions constant

A second feature that must present in the experiment in order to conclude that an apology causes people to have lower desires for revenge is called holding conditions constant. Holding conditions constant means that the only thing we allow to vary in the two conditions is the presence or absence of an apology. Everything else for the two groups is the same. Remember that scientists seek to isolate the variables they think impact behavior. By manipulating only whether an apology is present and holding all other potential variables constant, the researcher can test whether apologies influence vengeful behavior. Thus, in our example, the two scenarios are exactly the same except for one sentence about an apology.

The goal of experimental research is to understand the causes of people's behavior. When we manipulate an independent variable, randomly assign participants to conditions, and hold conditions constant, we are in a position to state that the independent variable causes any differences in the dependent variable. When we can confidently make this causal inference, we say that an experiment has internal validity.

Experimental designs are the most powerful designs for identifying cause-and-effect relationships (causal inferences) between variables. Thus, if your research question seeks to identify the causes of a relationship between variables, you should use an experimental design.