Final answer:
Experimental research is the appropriate method for testing the hypothesis that spaced practice leads to better retention than cramming because it allows the manipulation of the independent variable and measurement of the dependent variable in a controlled setup.
Step-by-step explanation:
To determine whether spaced practice results in better retention of material compared to cramming, the researchers should use experimental research. This is because experimental research involves the manipulation of variables to test for cause-and-effect relationships. In this scenario, the spaced practice and cramming are considered the independent variables that the researchers will manipulate, while retention of material is the dependent variable that they will measure.
By randomly assigning participants to either the spaced practice group or the cramming group, the researchers can control for external factors and focus on the effects of the practice methods. Furthermore, this approach enables the researchers to make causal inferences based on the differences in retention observed between the groups, provided that the experiment is designed and executed properly.
Other research methods like correlational research or observational research could reveal relationships between variables, but they would not provide evidence of causation. Case studies, while in-depth, are typically focused on a single subject or a very small group, and thus do not provide a broad enough basis for testing the hypothesized effect of practice methods on retention across a larger population.