In experimental designs with two variables, one is called the independent variable (IV) and the other is called the dependent variable (DV). The IV is the predictor variable whereas the DV is the outcome variable. Researchers manipulate the IV to study its effect on the DV.
The independent variable is also sometimes referred to as the treatment variable. In experimental designs, the independent variable is varied independently of participant's wishes. In other words, the independent variable is the variable manipulated by the researcher. In a simple experiment, there are at least two levels of IV (often low and high).
The dependent variable is the response that you are measuring (often some kind of score). Based on theories and/or previous research studies, the researcher predicts that response depends on the subject's exposure to the independent variable.
In trying to keep the independent and dependent variables straight, remember that subjects are exposed to the IV before the DV is measured. In thinking about the hypothesis for any particular experiment, remember that it could be restated as follows:
Independent Variable will cause a change in the scores on the Dependent Variable.
To illustrate, a researcher, Dr. Z, is interested in the effects of different presentations of course material on students' performance in a self-paced course taught at a distance. Dr. Z believes that by providing students with interactive on-line tutorials they will understand course material better than just using traditional learning materials such as textbooks. She randomly assigns students to one of two different versions of a self-paced course. Half of the students are given traditional course materials (textbook, assignments to complete, etc.). The other half of the students are given the same course materials, however, are also provided access to on-line interactive tutorials. Dr. Z hypothesizes that students given interactive tutorials will have higher final grades in the course than students given only traditional course materials. In other words, the course materials provided to students (IV) will affect the student's final grade in the course (DV). The type of materials given to students is the independent variable. It is the variable that is manipulated by the researcher. The student's final grade in the course is the dependent variable. It is predicted that exposure to the IV will ultimately affect the grades students achieve in the course (DV).