Final answer:
The researcher measured the dependent variable, which is the participant's cognitive functioning, to determine the effects of vitamin D supplements compared with a control group receiving a placebo.
Step-by-step explanation:
The researcher's test of participants' cognitive functioning after taking vitamin D supplements is a measure of a dependent variable. In the context of this study, the dependent variable is the outcome that the researchers measure, which in this case is cognitive functioning, to see if there is an impact caused by the manipulation of the independent variable—the intake of vitamin D.
In experimental research, the independent variable is what the researcher changes or controls to examine its effect on the dependent variable. The control group, which received the placebo in this study, helps ensure that the effects observed can be attributed to the vitamin D treatment rather than other factors. This is a classic experimental setup where one group (the experimental group) receives the treatment suspected to have an effect, and another group (the control group) does not, allowing for a comparison to see if there are any significant changes between the two groups.
This study likely employs a double-blind method where neither the participants nor the researchers know who receives the actual supplement or the placebo to prevent bias. Such careful control and manipulation of variables is essential to determine causal relationships in research experiments.