Chapter 10 Basic between-subjects designs The experimental design - is the general structure of the experiment between-subjects design. –Comparing groups that contain different subjects. –Later we will discuss within subjects designs.

Selecting and recruiting subjects –Random sampling Better external validity rarely happens Introductory psychology students

How many subjects should be in an experiment? Too few can be a problem –1) May not represent the population –2) May decrease power Human studies usually require participants than do animal studies –Why?

One Independent Variable: Two-groups designs –two versions two independent groups design two matched groups design

Two independent groups design Selecting the sample –Typically a convenience sample is used hypothesis testing –We have predicted that a manipulation will cause a difference between our otherwise equal 2 groups. –This can be accomplished without using a truly random sample

Assignment to groups Random assignment –every subject has an equal chance of being placed in either of the treatment conditions.

Random selection –external validity Random assignment –internal validity

An example of a two independent groups experiment –Loftus and Burns (1982)

Another independent groups experiment –Brady (1958)

Two types of two independent groups designs –1) Experimental group-Control group design –2) Two experimental groups design Experimental group – Control group design –experimental condition = treatment –control condition = no treatment Loftus & Burns (1982)

Two Experimental Groups design –participants are exposed to different levels of the Independent Variable. Holloway & Hornstein (1976) –Good news vs. Bad News in a waiting room

Matched groups designs –Page (1958) –This was performed with an independent groups design, but is a great candidate for a matched groups design –The effect of student comments on papers

This design is fine if you have a decent sized N. If small N - groups might be somewhat unequal at the start –use a matched groups design

Group 1Group 2 1 = 611 = 72 2 = 652 = 92 3 = 713 = 76 4 = 604 = 83 5 = 835 = 95 6 = 586 = 75 mean – 66.33mean – 82.17

A two-matched groups design will eliminate this problem How to match First rank order the scores according to their first paper –58 –60 –61 –65 –71 –72 –75 –76 –83 –92 –95

Now randomly assign one member from each pair to groups (flip a coin) Gp1 Gp mean =73.17 mean = 73.67

Rules about matching The closer the matching variable is to the dependent variable the better –1) match using actual dependent variable Grades and grades –2) match on closely related variable Zajonc (1969) –3) match on related variable IQ and grades

Methods of matching –Precision matching Same score –Range matching Specified range –Rank ordered matching All subjects included

When to use a matched groups designs –best when you have small numbers of subjects Disadvantage –If you match on an unimportant variable you will actually lose power fewer degrees of freedom

One independent variable with multiple levels Multiple Groups Design –Sometimes more than one group is required to test the hypothesis –Drug studies use dose response c`rves usually not a simple linear relationship