Methods Cozby Experimental Design 2

Pretest-Posttest Design • However, sometimes it is helpful to measure the participants on the DV both before and after the manipulation: – Small sample size (less than ~25 participants per cell) – Participant selection – Risk of mortality • Unfortunately, pretesting is more time consuming and costly. Plus, pretesting risks sensitizing the participant. Interpreting Experimental Results • Once you have measured the participants on the DV, you can compare the scores of the two groups: 9 8 7 6 5 Sympathy 4 3 2 1 0 Whining Control Expanding the Experiment... Expanding the experiment • Psychological phenomena are way too complicated to ever be explained with this basic design. Usually, experiments will increase: – the number of levels of the IV – the number of IVs. Increasing # of Levels of the IV Increasing the # of levels allows us to: 1. Detect curvilinear relationships: Whining (control, low, high) Sympathy (1-10) 2. Compare more than two groups: Persuasion (none, whining, screaming, begging, asking) Sympathy (1-10) Increasing the number of IVs • Designs with more than one IV are called factorial designs. • Suppose we want to know how the frustration level of the partner interacts with different persuasion techniques Persuasion (whining, screaming, asking) Sympathy (1-10) Partner’s Frustration (low, high) Factorial designs Terminology • Each IV is called a factor, and of course each will have at least 2 levels • An experimental design can be described by the number of IVs and the number of levels of each: Number of levels (IV1) X Number of levels (IV2)... Factorial designs • Thus, our study would be a 3 X 2 • This creates 6 conditions, or cells, that participants will be assigned to. Each cell of a design requires ~25 participants. Interpreting Factorial designs • Factorial designs yield two kinds of info: 1. Main effects Information about each of the IVs taken by itself. 2. Interactions Information about each of the IVs across levels of the other IV(s). Main Effects Remember the results from our first, basic design: 9 8 7 6 5 4 3 2 1 0 Whining Control For all practical purposes, this is a main effect for whining Main Effects Persuasion technique Screaming Asking 12 10 8 6 4 2 0 Whining Now we can combine persuasion with frustration: Hi Frustration Lo Frustration Main Effects & Interactions • Three ways to present information: 1. Bar graph 2. Line graph 3. Table See p. 185 Expanding some more... • Longitudinal designs are a special kind of expansion, in which the same measures are repeated at least twice, in order to assess how people change over time. • Thus, Time can be thought of as an additional IV, and the number of repetitions (Waves) are the levels of the variable.