BASIC BETWEEN-SUBJECTS DESIGN

CHAPTER OBJECTIVES:

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.

EXPERIMENTAL DESIGN

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?

BETWEEN-SUBJECTS DESIGN

Different subjects take part in each condition of the experiment

Comparison between the behaviors of different groups of subjects

BETWEEN-SUBJECTS 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

SELECTING AND RECRUITING 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

BETWEEN-SUBJECTS DESIGNS (1 IV, 2 GROUPS)

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

TWO INDEPENDENT GROUPS

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.

TWO INDEPENDENT GROUPS

When can 2 independent groups be used?

If there is only one independent variable If randomization can be assumed successful

TWO MATCHED GROUPS

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

TWO MATCHED GROUPS

How to match? Precision Matching – matched pairs have

identical scores Range Matching – matched pair fall in a specified

rangeNote: Some subjects have to be discarded Rank-order Matching – subject scores are ranked;

adjacent scores are paired.

TWO MATCHED GROUPS

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

BETWEEN-SUBJECTS DESIGNS (1 IV, MULTIPLE GROUPS)

BETWEEN-SUBJECTS DESIGNS (1 IV, MULTIPLE GROUPS)

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

ASSIGNING SUBJECTS

Use of block randomization

CHOOSING TREATMENTS

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.

PRACTICAL LIMITS

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.