CHAPTER 5Introduction to

Experimental Research

Chapter 5. Introduction to Experimental Research

Chapter Objectives

• Define a manipulated independent variable and identify examples that are situational, task, and instructional variables

• Distinguish between experimental and control groups

• Recognize the presence of confounding variables in an experiment and understand why confounding creates serious problems for interpreting the results of an experiment

Chapter Objectives

• Identify independent and dependent variables, given a brief description of any experiment

• Distinguish between manipulated independent variables and those that are subject variables

• Describe the interpretation problems that accompany the use of subject variables

Chapter Objectives

• Recognize the factors that can reduce the statistical conclusion validity of an experiment

• Distinguish between the internal and external validity of a study

• Describe the various ways in which an experiment’s external validity can be reduced

The meaning of Experiment• Experiment: a systematic research study in

which the investigator directly manipulates some factor, holds all other factors constant, and observes the results of the variation.(contrast with correlation)

• “investigating the effect of X on Y”

Essential Features of Experimental Research • Establishing independent variables (IVs)• Manipulated IVs : must have minimum of 2 levels• Situational: environmental features• Task: different problems to solve• Instructional: performing tasks in different ways

• Experimental groups• given treatment• Research Example given a golf ball and told it was a “lucky” ball

• Control groups• treatment withheld• Research Example given a golf ball and not told it was a “lucky”

ball

Essential Features of Experimental Research

• Controlling extraneous variables: variables which are not of interest to the researcher but which might influence the behavior being studied if not controlled for properly.• Confounds• Any uncontrolled extraneous variable• Covaries with the IV; results could be due to IV or to

confound• Distributed practice example

Essential Features of Experimental Research

• Measuring dependent variables (DVs)• DVs are any behaviors measured in an experiment• Review scales of measurement (Ch. 4)!

• Problems:• Ceiling effects• task is too easy, all scores very high, disguising any

differences• Floor effects• Task too difficult, all scores very low, disguising any

differences• Solution:• Task of moderate difficulty, determined through pilot

testing

Manipulated versus Subject Variables

• Subject Variables• Already-existing attributes of subjects in a study• Examples gender, age, personality characteristic

• Anxiety example• As a manipulated variable induce different degrees

of anxiety in participants• As a subject variable choose participants who have

different degrees of their typical anxiety

Manipulated versus Subject Variables

• Cannot draw certain conclusions when using subject variables• With a manipulated IV• Assuming no confounds IV causes DV

• With a subject IV• Groups may differ in several ways IV cannot be

said to cause DV• All that can be said the groups differ from each

other

Manipulated versus Subject Variables

• Using both manipulated and subject IVs• Bandura’s Bobo study (Box 5.2)• Manipulated type of exposure to violence• Subject gender

Independent: Type of aggression, gender

Extraneous:Emotional arousal, Proportion of aggressor to Bobo size

Dependent:Aggression score

The Validity of Experimental Research• Statistical conclusion validity

• Proper statistical analyses and conclusions

• Construct validity• Well-chosen and well-defined IVs and DVs

• External validity: the degree to which research findings generalize beyond the specific context being studied (subject pools, the college sophomore problem, the male problem)

• Internal validity- methodologically sound and confound-free (confidence that IV is directly responsible for results)

Threats to Internal Validity

• Studies extending over time (may have pretests)

• Example: Test-anxiety reduction• History – courses become Pass/Fail• Maturation • Regression to the mean• Testing and instrumentation- practice effects,

change in measurement instruments/coders• Importance of using a control group

Threats to Internal Validity• Participant problems• Subject selection: method/criterion for

participation produces a confound• The Brady study ulcers in executive monkeys

• Attrition• Loss of data• Subject selection problem

Summary

• Experimental research involves independent and dependent variable, in an effort to test the effects of the IV on the DV.

• We attempt to control for confounding variables to increase the internal validity of our study.

• We must consider other possible threats to internal validity as they pertain to our study.

• Once we identify IVs, DVs, and threats to validity, we design a study to control those threats.