Experimental Psychology PSY 433

1. Experimental PsychologyPSY 433 Chapter 3 Experiments – Kinds of Variables

2. Causal Relationship • Does a change in X cause a change in Y? • Three components: • Co‑variation of events • Time‑order relationship • Elimination of alternative causes.

3. Kinds of Variables • Independent variable (IV) • Dependent variable (DV) • Confounding variable – confound • Control variable • Random variable – not in the textbook • Subject variable

4. Independent Variable • The presumed "cause" of a behavioral effect or change in the DV. • Manipulated (varied) by experimenter. • IV has several levels selected by the experimenter. • Occurs, or can be "set up" before the DV is measured. • "Independent" of what the subject does.

5. Dependent Variable • A measurement of behavior -- also a measure of the size of the effect of the IV (the cause of behavior change). • What is recorded by the experimenter. • The behavior occurs after the IV is varied, and DV measures the change in behavior. • "Depends" on manipulation of the IV. • The DV does not have levels.

6. Two Kinds of Influence on DV • Systematic – affects the DV in the same way each time. • Can introduce bias into results. • When it occurs due to the manipulation of the IV we call it an “effect.” • Random – varies and thus affects the DV differently from observation to observation. • Can introduce “noise” into results. • Typically makes it more difficult to observe systematic influences.

7. Confounding Variable • Any variable, besides the IV, that can influence the DV. • A potential cause for the experimental effect, other than the IV. • An alternative explanation for observed findings in a study. • Any variable whose values change systematically across levels of the IV.

8. Alternative Explanations • Does chewing gum help students do better on exams? • What are the confounds? • This is another way of asking, what are the possible alternative explanations for a result? • Chewing gum requires energy because muscles are moving, so there is arousal. • Chewing gum releases sugar into the blood stream, providing more energy. • Sugar may be a reinforcer for learning.

9. Control Variable • A variable whose values remain the same across levels of the IV (e.g., room temp, light levels, time-of-day, etc). • A goal in experimentation is to control as many variables as possible, to eliminate their potential effect on the DV. • Eliminates both confounds and noise. • Except for the IV, each subject should have as closely similar an experience in the experiment as possible.

10. Random Variable • Variable whose values vary randomly in an unbiased way across levels of the IV. • Random variables are usually created by the process of random assignment to levels in the experiment.

11. Subject Variable • A personal characteristic • Sex, height, weight, age, education, ethnicity, socio-economic status, etc. • Sometimes called a “demographic” variable. • Data describing subject characteristics should be collected in every study. • Can be controlled or left to vary unsystematically (through random assignment to levels).