In an experiment, a researcher is interested in whether mood affects memory. She attempts to manipulate people’s mood with happy or sad music and then measures their performance. However, the researcher is not sure that the music is really causing people’s mood to change. In this case, what should the researcher do?
a. Control mood by holding it constant.
b. Control mood by matching it across treatment conditions.
c. Add a no-treatment control group in which mood is not manipulated.
d. Add a manipulation check by measuring mood.