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### IV. Independent, Dependent Variables, and Experimental Design

Before you proceed to do the experiment, we need to describe the design of the experiment. Specifically, we need to describe independent variables and dependent variables.

Independent Variables

Theories describe cause and effect relationships. Experiments test cause and effect relationships by using independent variables. Like all causes, independent variables have certain effects. For example, an experiment might test the effect of sleepiness on the ability to drive a car. Here, sleep time is the independent variable. An independent variable, like an algebraic variable, can have different values. For example, the sleep time variable could have 0, 4 or 8 hours of sleep. Specifically, the experiment would have a subject sleep 0 hours, 4 hours, or 8 hours before letting the person drive.

Our experiment will have two independent variable. The fiirst is type of picture change. Picture A & A' will differ in one or two ways: 1) a high-interest area will change; or 3) a low-interest area will change. An area was designated as high or low interests by having an independent observer simply described a picture. All the objects or areas of the picture mentioned the observer were designated as high-interest. Objects or areas not mentioned where designated as low-interest. The second is type of trial. There will be two types of trials: one type will have a blank field (or "gap") between each picture presentation (see figure 2 below) or it will not: there will be no gaps between picture presentations.

Dependent Variables

If independent variables are the "cause" part of an experiment, then dependent variables are the "effect" part. Here, the time it takes you to notice a change in a picture will be the dependent variable.

Overall Design, Procedure and Instructions

One trial will consists of the following events, for 1 minute or until you respond:

Figure 2. General procedure and instructions. Picture A will be presented for 240 ms, then a blank field for 80 ms, then picture A again (for 240 ms), and then the blank field (for 80 ms). Then the alternate picture, A', will be presented for 240 ms, then a blank field for 80 ms, then picture A' again (for 240 ms), and then the blank field (for 80 ms). Your task will be to detect if there is any difference between picture A and picture A'.