Dr. Z's experiment examined the effect of types of course material (IV) on students' grades in the course (DV). Note that in experimental designs, the independent variable can also be referred to as a factor. As you might have noted in Dr. Z's experiment, the independent variable or type of course materials factor consisted of two different levels (a. traditional course materials or b. traditional course materials accompanied by interactive tutorials). Every independent variable will have at least two different values (or levels) although it is common for some experiments to have multiple levels of the independent variable. In one-factor experiments, the different levels of the IV can also be referred to as treatments or conditions (McBurney & White, 2004). In identifying the levels of an independent variable, first ensure that you have correctly identified the independent variable itself. Next, consider how the independent variable varies. That is, what are the different values that the independent variable can assume? Remember that often different levels represent the absence or presence of something. For example, in Dr. Z's study, one group received the interactive tutorials (the tutorials were present) while the other group did not (the tutorials were absent). When there are two levels of an independent variable representing the absence and presence of a treatment or intervention, these are often referred to as the experimental group or condition and the control group or condition. The experimental group refers to the group who did receive the treatment (presence of the intervention). The control group is the basis of comparison to the experimental group, in that the control group does not receive the treatment or intervention.
In other cases, levels of the independent variable will represent degree of exposure. For example, suppose Dr. Z proposed that in order for interactive tutorials to be effective it is important that they are extensively used for all concepts presented in the course. As such, she decides to replicate her experiment by adding a new level to the independent variable. In this experiment, students are randomly assigned to one of three different levels of the independent variable. In the first condition, students are provided with only traditional course materials. In the second condition or level, students are provided with the traditional materials plus an interactive tutorial related to one of the more difficult concepts in the course. In the third level, students are given traditional materials along with four different interactive tutorials covering each of the four major concepts discussed in the course. In this experiment, there is still only one independent variable (supplemental course materials factor), however, this variable has three different levels representing degree of exposure to supplemental course materials (no exposure, one interactive tutorial, and four interactive tutorials).
Up to this point, we have described the independent variable as the variable that is manipulated by the researcher. However, there are cases in which the independent variable is not manipulated by the researcher but represent intact group membership (age, gender, etc.). In these cases the variables are called subject variables (McBurney & White, 2004). Subject variables represent variables of interest that cannot be manipulated or to which level of the independent variable subjects cannot be randomly assigned. These variables present some interesting and complex challenges and are the focus of non-experimental and quasi-experimental research designs discussed in other sections of the course. For the time being, our focus will be on true experimental designs, in which the researcher has control over how the independent variable is manipulated or changed and how subjects are assigned to the levels or conditions of the independent variable.