Overview of Experimental Research Design

8/11/2019 Overview of Experimental Research Design

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Experimental Research Design



Overview

Topics Appropriate to Experiments

The Classical Experiment

Selecting Subjects

Variations on Experimental Designs

An Illustration of Experimentation

"Natural" Experiments



Topics Appropriate to

Experiments

Projects with limited and well-defined concepts.

Projects that are explanatory rather than descriptive.

Studies of small group interaction.



Components of

Experiments

Three Pairs

Independent and dependent variables

Pretesting and posttesting

Experimental and control groups



Broad DistinctionsBetween subjects

Dependent measures taken one time

Data are independent (i.e., not correlated)

Within subjects

A “repeated measures” design

Dependent measures taken multiple times

Data are dependent



Design Types

Single factor (one-way)

Studies one independent variable

Multi-factor Studies multiple independent variables

May have several levels

Examples:

Two-way (e.g., 2 x 2)

Three-way (e.g., 2 x 2 x 2)

Time-series



Experimental and

Control Groups

Must be as similar as possible.

Control group represents what the experimental

group would have been like had it not been exposed

to the experimental stimulus.



Selecting Subjects/Cases

Probability sampling

Randomization

Matching



Randomization and

MatchingMay not know which variables will be relevant for

matching process.

Most statistics used to analyze results assume

randomization.

Randomization only makes sense if you have a largepool of subjects.



Preexperimental

Research DesignsOne-shot case study - single group of subjects is

measured on a variable following experimental

stimulus.

One-group pretest-posttest design - adds a pre-test

for the group, but lacks a control group.

Static-group comparison - includes experimental

and control group, but no pre-test.



Sources of Internal

InvalidityHistorical events may occur during the course of the

experiment.

Maturation of the subjects.

Testing and retesting can influence behavior.

Instrumentation



Sources of Internal

InvalidityStatistical regression of subjects starting out in

extreme positions.

Selection biases.

Experimental mortality - subjects drop out of the

study before it's completed.

Demoralized control group subjects.



Limiting External

Invalidity

Solomon four-group design

Posttest-only control group design



Solomon Four-group

DesignFour groups of subjects, assigned randomly:

Groups 1 and 2 are the control and experimental

group.

Group 3 does not have the pre-test.

Group 4 is only post tested (no stimulus; i.e. IV)



Solomon 4-group design

A di"erent design can allow us to look at the

e"ects of a pretest



R X O

R OR O X O

R O O

If these two groups are different, pretestsensitization is an issue.

Pre X Treatment interaction

If these two groups are different, there is atesting effect in general.

Solomon 4-group designIncluding a pretest can sensitize participants and

create a threat to construct validity. Combining

the two basic designs creates the Solomon 4-

group design, which can determine if pretest

sensitization is a problem:



Posttest-only Control

Group Design

Includes Groups 3 and 4 of the Solomon design.

With proper randomization, only these groups are

needed to control the problems of internal invalidity

and the interaction between testing and stimulus.



"Natural" Experiments

Important social scientific experiments occur outsidecontrolled settings and in the course of normal social

events.

Raise validity issues because researcher must take

things as they occur.



Experimental Method

Strengths:

Isolation of the experimental variable over time.

Experiments can be replicated several times using

di"erent groups of subjects.



Experimental Method

Weaknesses:

Artificiality of laboratory setting.

Social processes that occur in a lab might not occur

in a more natural social setting.

Overview of Experimental Research Design

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