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 genderIndependent: 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.