To investigate the effects of a particular study method on student performance, two different methods are tried, each with a different group of subjects. If only the experimenter knows which method is under investigation, the procedure being used is described as?

single-blind.

Explanation:

Single-blind study is a human clinical trial method where only the subject examiner knows what is being used as a variable at any given time. It is commonly used as a criterion for validating quantitative experimental practices in science.

An example of this can be seen in the question above, where only the researcher knows which method is under investigation, while the research participants know nothing.