ch 8: experimental design ch 9: conducting experiments
TRANSCRIPT
Ch 8: Experimental Design
Ch 9: Conducting Experiments
Uses of Experimental Research
Test hypotheses derived from theories
Test the effectiveness of a treatment or program
Examine the causes of behavior
Conducting Experimental Research
Manipulate independent variable to see effect on dependent variable
Compare groups in terms of their scores on the dependent variable
All other variables kept constant through direct experimental control and/or randomization
Independent Variable
This variable is manipulated (controlled) by the experimenter and has at least two different levels (conditions)
1000 mg
0 mg=placebo
Manipulating the Independent Variable Straightforward manipulations
Amount of substance administered Written instructions Verbal material Visual material
Staged manipulations Often employ confederates Events are staged or manipulated to:
Create some psychological state Simulate some situation that occurs in the real world
Dependent Variable
This variable is measured by the experimenter and is used to determine the effect of the independent variable
no pain mild moderate severe
Measuring the Dependent Variable
Types of measures• Self-report measures• Behavioral measures• Physiological measures
Sensitivity of the Dependent Variable
The independent variable can appear to have no effect on the dependent variable when there is a• Ceiling effect—participants quickly reach
the maximum performance level• Floor effect—a task is so difficult that
hardly anyone can perform well
Posttest-only Design Obtain two equivalent groups of
participants (R=random assignment) Introduce the independent variable Measure the effect of the independent
variable on the dependent variable
Posttest-only Design Involves random
assignment of individuals to the different conditions of the independent variable
Random assignment A method for placing subjects in conditions prior
to implementing the independent variable Every individual has the same chance of being
placed in a given condition
Pretest-posttest design
Same as a posttest-only design but adds a pretest before the experimental manipulation
Allows the researcher to ascertain if the groups are equivalent at the beginning of the experiment
Example: Are kids healthieron some dimension (weight,stamina) after going throughan athletics program?
1. Assess equivalency with small sample size
2. Assess mortality (attrition or dropout factor)
Advantages of the pretest-posttest design
Mortality Mechanical subject loss: equipment
failure or experimenter error leads to loss
Selective subject loss: some characteristic of participant is responsible for loss
Disadvantages of the pretest-posttest design
• Time consuming• Awkward to administer• Sensitizes participants to what is being studied Demand characteristics:
Cues and information a
participant uses to guide
their behavior in a
psychological study
“Is research on cultural acceptance?”
Demand characteristics
o Possible solutions:• disguise pretest• embed the pretest in another measure (filler questions)• concealed observation
Placebo control group —used to assess whether participants’ expectancies contribute to the outcome of an experiment
Internal validity
Occurs when we are able to confidently state that the independent variable caused the differences we observe
Causal inferences can be made when internal validity is present
Confounding
This occurs when the variable of interest and a different potential independent variable are allowed to covary
Represents an alternative explanation for a study’s findings
Threatens internal validity
Other Threats to Internal Validity
Intact groups Extraneous variables Experimenter effects
Intact groups
This occurs when groups are formed prior to the start of an experiment
Selection differences: systematic ways in which people can differ
Selection differences
Characteristics of people that differ or vary:Physical characteristics: sex, raceSocial characteristics: ethnicity, religion,
marital statusPersonality characteristics: extraversion,
emotional stabilityMental health characteristics: depression,
anxiety
Extraneous variables Other practical considerations that
may confound an experiment
Experimenter effects Biases that occur when
experimenters’ expectancies regarding the outcome of the experiment influence their behavior toward participants in different conditions
Control by automating procedures as much as possible
Double-blind experiment A procedure in which
both the participants and the experimenters are unaware of which condition is being administered
Controls for both demand characteristics and experimenter effects
Experimental control techniques
Methods for ruling out other possible causes for the outcome
Assigning Participants to Experimental Conditions
Independent groups design• Participants randomly assigned to conditions• Participants are in only one group
Low-meaningful
High-meaningful
15 randomly assigned
participants
Another 15
randomly assigned
participants
Meaningfulness
Assigning Participants to Experimental Conditions
Repeated measures design• The same participants are in all of the groups
Low-meaningful
High-meaningful
15 participants The SAME 15
participants
Meaningfulness
Repeated Measures Design
Advantages Fewer participants Extremely sensitive to statistical differences (more
likely to detect an effect of the IV on the DV) Disadvantages
Order effects Practice effects Fatigue effects Contrast effects
Minimizing order effects Counterbalancing
1. Complete counterbalancing—all possible orders of presentation are included in the experiment
Matched pairs design
Ensures groups are equivalent on the matching variable prior to the IVMatch participants on a
particular characteristicAfter matching, randomly
assign to experimental conditions