Pratab, a research scientist, conducted a study to examine the effect of marijuana on memory. He selected four groups of male subjects that were comparable in age and education. Each subject was given a different oral dose of marijuana in a brown cookie. After consuming the cookie, the subjects performed various tasks, including memorizing lists of 10 unrelated words. One week later, the subjects returned and were asked to recall the words from the lists. The study aimed to test the hypothesis that marijuana impairs memory, with marijuana as the independent variable that was manipulated and human memory as the dependent variable that was measured.