B ASIC B ETWEEN -S UBJECTS D ESIGN

C HAPTER O BJECTIVES : 1. Learn how subject are assigned to conditions of a between-subjects experiment and what random assignment accomplishes. 2. Learn the elements of simple two group designs and how to control for confounding 3. Learn how to conduct experiments with more than two groups 4. Understand why one design is selected over another.

E XPERIMENTAL D ESIGN General structure of the experiment Number of treatment conditions Same or different subjects in different treatment conditions Determined by The nature of the hypothesis Information the researcher is seeking Practical problems in running the experiment Questions to consider How many independent variables are there? How many treatment conditions are needed? Will we use same or different subjects?

B ETWEEN -S UBJECTS DESIGN Different subjects take part in each condition of the experiment Comparison between the behaviors of different groups of subjects

B ETWEEN -S UBJECTS DESIGN Two Groups Two Independent Groups Experimental group-Control group 2 Experimental group Two Matched Groups Experimental group-Control group 2 Experimental group Multiple Groups

S ELECTING AND R ECRUITING SUBJECTS The more the sample resembles the whole population, the more likely is it that the behaviors of the sample mirrors that of the population. How many subjects are needed for between- subjects design? at least 15 to 20 subject in each treatment condition More comfortably, 30 per treatment

B ETWEEN -S UBJECTS D ESIGNS (1 IV, 2 G ROUPS ) How were the subject assigned? Two Independent Groups Matched Groups What are the levels of IV? Experimental group – Control group Design Two Experimental group Design

T WO I NDEPENDENT G ROUPS Grouped using random assignment What are the possible treatment conditions (level of the IV) for 2 independent groups? Experimental group-Control group ; Experimental condition-Control condition; Treatment-No treatment; however, “no treatment” does not really mean “no treatment” all the time. Two Experimental group (No Control group) – gathers more precise information; different values of the IV.

T WO I NDEPENDENT G ROUPS When can 2 independent groups be used? If there is only one independent variable If randomization can be assumed successful

T WO M ATCHED G ROUPS Does/must/can not use random assignment 2 groups of subjects assigned by the researcher by equating them on a particular characteristic that might affect the DV Matching can be done before or after the experiment

T WO M ATCHED G ROUPS How to match? Precision Matching – matched pairs have identical scores Range Matching – matched pair fall in a specified range Note: Some subjects have to be discarded Rank-order Matching – subject scores are ranked; adjacent scores are paired.

T WO M ATCHED G ROUPS When can 2 matched groups be used? Presence of a strong extraneous variable Need for more efficient measures of the IV’s effect When there is a very small number of subjects

B ETWEEN -S UBJECTS D ESIGNS (1 IV, M ULTIPLE G ROUPS )

Sometimes, it takes more than two treatment conditions to make a good test of a hypothesis When amount or degree of the IV is important Different values of the same variable can produce different effects May give complete understanding of how the variable works Each group is run though a different condition One treatment condition may be the control Usual multiple independent groups Matching is possible

A SSIGNING S UBJECTS Use of block randomization

C HOOSING T REATMENTS How does one choose how many treatments to use? Theoretical basis Equal or proportional intervals What will I gain by adding these extra conditions to the experiment? As a general rule, select the simplest design that will make an adequate test of your hypothesis.

P RACTICAL L IMITS More subject are needed Takes more time More complicated statistical procedures Use of Pilot study (mini experiment) pretest selected levels of an IV before conducting the actual experiment.