Handout #9:  - Experimental Research

 

 

INTRODUCTION

 

Wiersma (2000) defines an experiment as a research situation in which at least one independent variable, called the experimental variable, is deliberately manipulated or varied by the researcher to test its possible effect on another. In other words experiments test cause and effect relationships. A well-designed experiment should have good experimental validity.  For it to have good internal validity, an experiment should establish without doubt that the experimental variable is the cause of the effect by controlling the effects of all other factors.  For it to have good external validity it should be possible to generalise the findings to a range of conditions.  Internal validity can be improved by:

 

a)         Keeping factors constant.      In an experiment to see the effect of sales pitch on the sale value of houses, the background of the subjects (persons taking part in the experiment) as well as the design of the houses is important.  An experimenter could select two groups of persons, expose one set to the sales pitch (the treatment) and not expose the other set and compare the price both sets are willing to pay.  The background of both groups should be similar and the design of houses should be similar or both background and design would have an effect on sale value and would mask or confound the effect of sales pitch.  Keeping factors constant leads to problems with external validity, as the findings would only be applicable to the conditions used.  To improve external validity the experiment should be repeated using other conditions.

 

b)         Including a control group.      In the experiment described above the group that is not exposed to the sales pitch is known as the control.  The control group provides a basis for comparison.  If an effect is observed in the test subjects and not in the control it can be assumed that the effect is due to the treatment if both groups are similar.

 

c)         Random assignment.    Random assignment of subjects to the groups is used to ensure that the groups are similar.  In random assignment each individual has an equal chance of being placed in either group so the characteristics should be spread evenly between both groups. Experimental studies fall under three categories: pre-experimental designs, experimental designs and quasi-experimental designs.

 

PRE-EXPERIMENTAL DESIGNS

 

1.      The One-Shot Experimental Case Study

 

 Treatment – Observation

 

This is not to be confused with case studies in qualitative research.  In this design a group is exposed to a treatment and an observation made.  It is a very poor design as factors are not kept constant, there is no control and no random assignment. It is not certain whether a change occurred as no observation was made before the treatment.

 

2.      The One-Group Pretest – Posttest Design

 

Observation – Treatment – Observation

 

In this design an observation (pretest) is made before the treatment is applied and after the treatment is applied (posttest).  The effect of other factors cannot be ruled out.  However, it can show that a change occurred.

 

3.      The Static-Group Comparison

 

Group 1            Observation      – Treatment –   Observation

Group 2                                                                        Observation

 

In this design a control group, to which a final observation is made, is used.  As no attempt is made to ensure that both groups are similar, the effects of other factors is not ruled out.

 

TRUE EXPERIMENTAL DESIGNS

 

1.      The Pretest – Posttest Control Group Design

 

Random                        Group 1            Observation      – Treatment –   Observation

Assignment                   Group 2            Observation                                          Observation

 

In this design subjects are randomly assigned to both groups and observations are made before and after the treatment is administered to the test group.  Cause and effect is established.

 

2.      The Posttest – only Control Group Design

 

Random                        Group 1            Treatment –      Observation

Assignment                   Group 2                                    Observation

 

In this design the pretest observation is removed.  In some experiments a pretest may influence the results and so the pretest is removed.

 

3.   The Solomon Four-Group Design

 

Random                        Group 1            Observation      – Treatment – Observation

Assignment                   Group 2            Observation                              Observation

                                    Group 3                                    - Treatment –    Observation

                                    Group 4                                                            Observation

 

Instead of removing the pretest the effect of the pretest may be assessed by adding two other groups.  This design is expensive.

 

4.   Factorial Experimental Designs

 

The Solomon Four –Group Design is an example of a factorial design in which the effect of more than one independent variables (in this case two; treatment and pretest) are measured.  Factorial designs allow for interactions between independent variables to be assessed.

   

QUASI-EXPERIMENTAL DESIGNS

 

1.   Non-randomised Control Group Pretest-Posttest Design

 

            Group 1            Observation      – Treatment – Observation

            Group 2            Observation                              Observation

 

This is similar to the pretest posttest control group design except that subjects are not randomly assigned.

 

2.         Time-Series Experiments

 

      Obs       Obs           Obs      Treatment         Obs      Obs      Obs

 

In time series designs several observations are made before and after the treatment.  A control group may or may not be used.

 

BIBLIOGRAPHY

 

Leedy, P.D. & Ormrod, J.E. (2001) Practical Research – Planning and Design (7th ed.). New Jersey:  Merrill        Prentice Hall.

Wiersma, W. (2000). Research Methods in Education – An Introduction. (7th ed.) Boston: Allyn and Bacon.