Post on 19-Dec-2015
EXPERIMENTAL DESIGNS
Criteria for Experiments Independent, Dependent, and
Confounding Variables Types of Experimental Designs Threats to Internal Validity Threats to External Validity
Criteria For Experimental Designs
Cause: experimenter manipulates a variable
Comparison: more than one condition Control: extraneous variables are
eliminated
Confounding Variable variable other than IV and DV which
changes between conditions control variable: potential confounding
variable that is controlled
Threats to Internal Validity:Individual Differences
Systematic differences between individuals in different groups
Strategies random assignment matched groups within subjects design
History
Events outside the experiment Most likely when conditions are measured
at different times with long delays Strategies
Decrease time between conditions Add a control group measured at same times
Maturation
Physical changes related to aging Particular problem for within-subjects
designs Strategies
decrease time between measurements add a control group measured at same times
Instrumentation
Changes in the measuring instrument or equipment
Strategy Use standardized administration
Attrition
Participants drop out of the study at different rates for the different conditions
Strategies Check attrition rates across groups Compare participants who drop out to those
who stay in
Diffusion of Treatment
Information about the purpose of the study is shared with future participants
Strategies Short time span between participants Use debriefing to request that participants do
not share information about the study
Demand Characteristics
Cues from the experimenter or research procedure about what behavior is desired
Strategy Single-blind procedure
Experimenter Effects
Experimenter’s expectations affect measurements
Strategy Double-blind procedure
Floor and Ceiling Effects
Measuring instrument is not sensitive enough Floor effects Ceiling effects
Strategy Check sensitivity of instrument prior to
experiment
Regression to the Mean
When measured twice, scores on the second testing tend to be closer to the mean
Statistical phenomenon due to chance Strategy
Don’t select participants for groups based on extreme scores
Use an equivalently selected control group that does not get the treatment
Order Effects
Also called Testing or Repeated Testing Effects of repeated measurements
Fatigue effects Practice effects Carryover effects
Strategy Counterbalance order of conditions
How Counterbalancing Works
Change the order of conditions Order effects will still exist but will affect
all conditions equally This prevents order effects from being
confounding
Complete Counterbalancing
Each possible order of conditions is used for an equal number of subjects
If your conditions are A,B, and C, 1/6 of participants will get each order: ABC CAB
ACB CBA
BAC BCA
Latin Square Counterbalancing
Each condition is presented in each position for an equal number of subjects
Controls for practice and fatigue effects
Example Latin Square
1st 2nd 3rd 4th
1/4 get A B C D
1/4 get B C D A
1/4 get C D A B
1/4 get D A B C
Balanced Latin Square
Latin square with additional requirement that each condition precedes and follows every other condition equally often
Controls practice and fatigue effects Controls simple carryover effects
(involving effect of a single condition)
Balanced Latin Square
1st 2nd 3rd 4th
1/4 get A B D C
1/4 get B C A D
1/4 get C D B A
1/4 get D A C B
Randomized Counterbalancing
Used when there are multiple stimuli tested for each condition
Put the stimuli in random order for each participant