Experimental research

1. Lecture Outline
 Introduction to experimental research
 Introduction to correlational research
 Three types of research validity
 Class Activity

2. Experimental Research
Overview and Major Features
 An independent variable (IV) is manipulated
 A dependent variable(s) (DV) is measured
 Many basic experiments consist of two levels of the independent
variable
 experimental group
 control group
 Control over extraneous variables
 holding constant
 randomizing effects
 A causal relationship between the independent and dependent
variables can be established

3. Basic Goal of Experimental Design
 Manipulate possible values of the IV
 Observe behavioral changes as indexed by values of the
DV

4. Basic Characteristics of Experimental Designs
 Subjects are randomly assigned to research groups
 A given participant must be equally likely to be assigned to any of the
given groups
 Random number table
 Flip a coin
 In experimental designs, a causal relationship between the
independent and dependent variables can be established

5. Independent Variable
 Differing values of the IV are called “levels”
 May vary quantitatively
 10 mg day
 20 mg day
e.g. here are 2 levels of variable dosage
 May vary qualitatively
 Zoloft
 Imipramine
 Prozac
e.g. here are 3 levels of the variable medication type

6. •An operational definition is a clearly defined set of
procedures for measuring or manipulating the construct of
interest.
•The key to an acceptable operational definition is that the
procedure is specified precisely enough to allow replication by
others.
Examples:
quality of memory -- accuracy of recall in a certain task
depression -- Beck Depression Inventory (survey) score
arousal -- galvanic skin response (conductivity of the surface
of the skin)
Operational Definitions

7. She is a good car
salesperson
She:
1. Sells many cars
2. Points out + features
3. Is nice
4. Sells only good cars
Verbal Statement Operational Definition
Operational Definitions: Examples

8. Stress
1. Environment/Number of stressors
a. Number negative Life events
b. Number of Examinations
2. Self-report
Measurable Nervous mood
3. Behavior
a. number of fidgets/minute
b. Psychophysiological
responses elevated
Verbal Statement Operational Definition
Operational Definitions: Examples

9. Identify Key Experimental Features
 Social loafing refers to people’s tendency to slack on group tasks.
In one study on social loafing college participants cheered alone,
and in groups of 2, 4, or 6 ppl. They were told to cheer as loud
as possible at specific times so researchers could determine how
much noise people made in social settings. The amount of noise
made by each participant dropped as group size increased.
 1) What is the IV?
 2) What is the DV?
 3) Why is random assignment important here?

10. Correlational Research: Major Features
 No independent variables are manipulated
 Two or more variables are measured and a relationship established
 Correlational research does not show causality
 Don’t confuse statistics with research design
 Correlation coefficients ( a statistic) can be used in correlational or experimental
research designs (although they are more commonly used in correlational designs)

11. Correlational Research
 2 Variables
Positive and Linear Negative and Linear

12. Example of Non-Linear Relationship
Yerkes-Dodson Law
Performance
Arousal
Better
Worse
Low
High

13. Correlation Does Not Imply Causation
Causality – must demonstrate that changes in one
variable can only be due to influence of the other
variable
 Directionality of Effect Problem
 Third Variable Problem

14. Directionality of Effect Problem
X Y
X Y
X Y

15. X Y
X Y
Class
Attendance
Higher
Grades
Class
Attendance
Higher
Grades
Directionality of Effect Problem

16. X Y
X Y
Aggressive Behavior Viewing Violent TV
Aggressive Behavior Viewing Violent TV
Aggressive children may prefer violent programs or
Violent programs may promote aggressive behavior
Directionality of Effect Problem

17. Third Variable Problem
X Y
Z

18. Class Exercise
Identify the
third variable
that influences both X and Y

19. Third Variable Problem
Number of
Churches
Crime
Rate
+

20. Third Variable Problem
Class
Attendance
GPA
+

21. Third Variable Problem
Ice Cream
Consumed
Number of
Drownings
+

22. Measurement Considerations
 Variables should be measured accurately and consistently.
 Accuracy
 Validity
 Bathroom scale gives correct weight
 Consistency
 Reliability
 Bathroom scale gives same weight consistently

23. Evaluating Research: 3 Validities
 Validity
 How accurate is the information from our research?
 Three validities
 Construct validity
 Internal validity
 External validity

24. Construct Validity

25. Construct Validity
 The degree to which the construct can be inferred from the operational
definition of that construct
 Never one perfect operationalization
 Schacter and Singer (1962)
 Two-factor theory of emotion:
 Emotions require arousal and label
 Factor 1: AROUSAL
 Participants received epinephrine shots and either
informed them of side-effects or not
 Factor 2: LABEL
 Participants around "euphoric" or "angry“
confederate

26. How Valid are These Manipulations?
 Euphoria
 Confederate shoots rubber bands, plays with hula hoops and crumples up
paper and practices hook shots into wastebasket
 Anger
 Confederate complains about the injection, adds negative comments
about the questionnaire they are filing out and ends up ripping up the
questionnaire.

27. Simplified Results
Euphoria
Condition
Anger
Condition
Informed of
epinephrine effects
Uninformed of
epinephrine effects

28. Internal and External Validity
 Internal validity
 The extent to which the observed effect is caused only by the
experimental treatment condition
 The ability to draw conclusions about a causal relationship from our data
 Experiments usually high in this
 External validity
 The extent to which the results can be applied to and across different
persons, settings and times
 AKA generalizability
 Experiments often low in this

29. Generalizing From the Lab Setting
 Mundane realism
 The extent to which an experiment is similar to real-life situations
 Psychological realism
 The extent to which an experiment triggers relevant psychological and
group processes
 Key point
 Lab experiments not automatically artificial
 When an experiment lacks mundane realism it may be very realistic in
terms of psychological realism

30. Assess Internal & External Validity
 Does lighting affect productivity?
 Study 1:
 Give people a production task in a laboratory. The
lab room is either bright or dim. Measure and
compare their productivity (how quickly and
accurately they perform the task).
 Study 2:
 Give employees in a well-lit office and those in a dim
office a production task. Measure and compare their
productivity (how quickly and accurately they
perform the task).

31. The Tradeoff
 Real life situations
 High external validity
 Hard to rule out other explanations
 Low internal validity
 Lab settings
 High internal validity
 Often artificial and findings can’t be generalized
 Low external validity

32. Class Exercise
 Hypothesis and Operational Definition Exercise