1. Experiments Observational Study – observes individuals and measures variables of interest but does not attempt to influence the responses. Experiment – deliberately imposes some treatment on individuals in order to observe their response. Treatment – an experimental condition applied to the units.

2. Experiments Experimental Units – individuals on which the experiment is done. Subjects – what human experimental units are called.

3. Experiments Response Variable – measures an outcome of a study. Also referred to as the “y” variable. Explanatory Variable – attempts to explain the observed outcomes. Also referred to as the “x” variable.

4. Experiments Lurking Variable – a variable that is not among the explanatory or response variables in a study and yet may influence the interpretation of relationships among these variables. Confounding – variables are confounded when their effects on a response variable cannot be distinguished from each other. Confounded variables may be explanatory or lurking. Example p. 270.

5. Experiments Factors – the explanatory variables in an experiment. Levels (treatments) – different values of each factor. Example p. 290

6. Experiments Placebo – a dummy pill or treatment that looks and tastes like the treatment but has no active ingredients. Placebo Effect – Response to a dummy treatment. Control Group – the group of patients who receive the placebo. The control group allows us to control the effects of outside variables on the outcome.