What do we call the variable an experimenter manipulates in order to determine its effects in an experiment?

You overhear psychology students preparing for a test. They note that one of the research methods often "fails to generalize." Which method is the most likely topic of this discussion?

a) correlation b) case study c) experiment d) naturalistic observation

__________ is associated with a decreased risk of Alzheimer's disease

A) Taking ginko biloba B) Having Down syndrome C) Taking folic acid D) Having a higher education