A clinical psychologist is interested in the effect that anxiety has on the ability of individuals to learn new material. She is also interested in whether the effect of anxiety depends on the difficulty of the new material. An experiment is conducted in which there are three levels of anxiety (high, medium, and low) and three levels of difficulty (high, medium, and low) for the material, which is to be learned. Out of a pool of volunteers, 15 low-anxious, 15 medium-anxious, and 15 high-anxious participants are selected and randomly assigned five each to the three difficulty levels. Each subject is given 30 minutes to learn the new material, after which the participants are tested to see how much they learned. Which statistical test should the researcher use to analyze the data?
a.) chi square
b.) independent samples t-test
c.) one-way analysis of variance (ANOVA)
d.) two-way analysis of variance (ANOVA)