Experimental Designs

Leedy and Ormrod, Ch. 10

Introduction Experiments are part of the traditional

science model Involve taking “action” and observing

consequences of this action Can collect data using rigorous control A good example of a sophisticated

experiemntal design experiment is Albert Bandura’s Bobo doll study: “Transmission of Aggression Through Imitation of Aggressive Models” (1961) Go to this link to read this classic paper:

http://psychclassics.yorku.ca/Bandura/bobo.htm

Introduction (cont.)

Excellent for hypothesis testing and establishing causality

For explanatory research Small group interaction Two types:

Laboratory settings (traditional) Used in psychology

Natural settings (field experiments) More typical in sociology

The classical experiment Has three pairs of components:

Independent and dependent variables IV is manipulated DV is observed for change

Pre-testing and post-testing To compare variation in DV before and after

treatment

Experimental and control groups Experimental group receives “treatment” and is

compared to control group (no treatment)

Experimental Subjects

Probability sampling if possible Random assignment essential to ensure

no difference between experimental and control groups

Sometimes “matching” used Individuals are matched on important

characteristics Problem of “volunteer” subjects

“Pre-experimental” Designs

No random assignment used A. One shot case study:

Tx (IV) Obs (DV)

B. One group pre-test-post-test design: Obs1 Tx Obs2

C. Static group comparison: Exp. Grp. Tx Obs

Ctrl. Grp. Obs

“True Experimental” Designs

Use random assignment D. Pretest-Post-test control group design

(classical experimental design):Exp. Obs1 Tx Obs2

Ctrl. Obs1 Obs2

E. Post-test only control group designExp. Tx Obs

Ctrl. Obs

“True Experimental” Designs (cont.)

F. Solomon Four-Group design (Bandura’s Design):Exp. Obs1 Tx Obs2

Ctrl. Obs1 Obs2

Exp. Tx Obs2

Ctrl. Obs2

Problems of Internal Validity

Is change in DV actually caused by IV?

A. Problems concerning Subjects1. Selection bias2. Experimental Mortality3. Rivalry4. Demoralization

Problems (cont.) B. Problems with experimental procedure

1. Testing2. Instrumentation3. Causal time order4. Diffusion5. Compensation to control group

C. Problems with timeHistoryMaturation

The Double-Blind Experiment

Neither researchers or subjects know who is experimental group

To reduce experimental bias

The Hawthorne Effect:

The Hawthorne effect – refers to change in behaviour produced by the “stimulus” of being singled out and made to feel important during the experimental process.

First observed 1925-1928 at Hawthorne plant in Illinois during series of experiments on effects of lighting on worker productivity

Explanation for increase provided by Professor Elton Mayo (“founder” of human relations theory)

The Hawthorne Experiments Four experiments:

A. Used Three experimental groups and no control group. Found productivity increased when lighting improved.

B. Experimental and control group used. Experimental group had increased lighting. Both groups showed increased productivity.

C. Experimental and control group used. Experimental group had decreased lighting. Both groups showed increased productivity.

D. Two women studied under varying lighting conditions. Subjects increased productivity according to which lighting conditions (more or less) researchers said were “good”

Other Problems:

External validity or generalizability of experimental research

Modelling Effect Sample problems

Volunteers vs non-volunteers have different characterisitics

Strengths and Weaknesses of Experimental Design Strengths

Establishing causalityControlLongitudinal researchReplication

WeaknessesArtificialityExperimenter effectSample size

The Field Experiment

Used in sociology Natural setting Uses statistical control For an excellent example of a field

experiment, read “The Impact of Television: A Longitudinal Canadian Study” by Tannis Macbeth Williams To read a summary of this work, go to:

http://world.std.com/~jlr/comment/tv_impact.htm

Strengths and Weakness of Field Experiments Strengths

Takes place in “real world”Very good generalizability

WeaknessesLack of internal validityDifficult to establish causalityTime consumingSubject to researcher bias