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Experimental DesignsExperimental DesignsHOW DO WE FIND
THE ANSWERS
?
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Characteristics of Experimental DesignsCharacteristics of Experimental Designs
Manipulation of one or more factors (Independent Variables)
Measurement of the effects of manipulation (Dependent Variables)
Validity Are we in control?
Reliability Can the results be replicated?
Sensitivity Are we measuring what we want to measure?
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Validity (Internal)Validity (Internal)
amount of control over experimental conditions allows conclusion that the IV causes an effect on
the DV allows exclusion of other variables causing an
effect on the DV
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Internal ValidityInternal Validity
Challenges to Internal Validity Using intact groups
(such as classes of students) Not balancing extraneous variables
(individual differences) * hypnosis volunteers early or late in term
Subject Loss mechanical subject loss (equipment failure) selective subject loss (related to paradigm?)
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Validity (External)Validity (External)
Can findings be generalized to other species to other individuals to other settings or situations to other conditions
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Validity (External)Validity (External)
For some lab experiments we do not establish external validity
Is external validity needed? Mook (1983) argues that external validity is
irrelevant if we are testing a specific hypothesis in a laboratory setting
Lab experiments typically try to test a specific hypothesis instead of imitating a typical situation
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Validity (External)Validity (External)
External validity needed when results are to generalized to a population
External validity requires a representative sample
Partial replication (repeating some but not all of the experimental conditions) can provide evidence for external validity
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SensitivitySensitivity
Is our measure appropriate for the effect we are looking for?
Are we measuring enough of the effect? Are we measuring too much of the effect
(even if we get an effect, is it meaningful?)
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Issues of ControlIssues of Control
Methods of Control Manipulation Holding conditions constant Balancing
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ControlControl
Manipulation Systematic varying of an Independent
Variable
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ControlControl
Hold Conditions Constant Make the IV the only variable the
differentiates between the groups Example: Use only males to hold the gender
effect constant
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ControlControl
Balancing Technique used to control for individual
differences of participants Used in independent groups designs Insures that all groups are equivalent in
areas such as age, motivation, sex, intelligence, etc.
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Independent Groups DesignIndependent Groups Design
Each group represents a different condition Conditions are defined by the level of the
IV Groups are formed by participants being
assigned to conditions Nature of group formation makes balancing
a major consideration of control
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Random Groups DesignRandom Groups Design
Groups are formed prior to introducing the IV
Subjects are sampled in such a way that the selection of one subject in no way influences the selection of another subject
All subjects have an equal chance of being in any given group
May be accomplished by random selection or random assignment
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Random SelectionRandom Selection
Requires a well defined population Requires randomization processes for
selection of subjects Subjects are randomly selected for each
group
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Random AssignmentRandom Assignment
Used when random selection is not possible Most samples are accidental and not from
well defined populations (Intro Psyc students)
Random assignment is then used to randomize subjects into different groups instead of random selection
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Random AssignmentRandom Assignment
Block Randomization Most often used for random assignment Have number of blocks = number of
subjects in each condition Randomize conditions in each block Assign subjects to each condition in each
block until all blocks are filled
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Matched Groups DesignMatched Groups Design
Used when comparable groups is required Instead of random assignment
the researcher makes the groups equivalent by matching the subjects in each group
Most useful if a good matching task is used
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Matched Group DesignMatched Group Design
Example 1) pretest for dependent variable (BP) 2) match subjects by BP level and group by the
number of conditions 3) randomly assign to conditions 4) compare BP of subjects by condition at
posttest
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Natural Groups DesignNatural Groups Design
Subjects are selected based on levels of IV Used when impossible to manipulate IV
age, gender, personality traits, etc. Used when not ethical to manipulate IV
married, divorced, widowed, etc.
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Repeated Measures DesignRepeated Measures Design
One group of subjects Subjects receive all levels of the IV Eliminates problem of Individual Differences Reduces the number of subjects required
Counterbalancing necessary for control
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CounterbalancingCounterbalancing
Counterbalancing necessary to control practice effect. ABBA design optimal (IVs A & B)
ABBA (complete counterbalance)
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CounterbalancingCounterbalancing
Problems with ABBA design Each subject has to complete all presentations of IV
ABBA As IV levels increase design becomes unmanageable
IVs A, B, & C ABCACBBACBCACABCBA
(Complete Counterbalance)
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CounterbalancingCounterbalancing
Alternate to complete ABBA design Use Incomplete design
½ of group receives conditions AB ½ of group receives conditions BA
IVs ABC (complete design) ABCACBBACBCACABCBA
(Complete Counterbalance – each subject receives 18 conditions) ABC ACB BAC BCA CAB CBA
(Incomplete Counterbalance – each subject receives 3 conditions)
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Design Problems and SolutionsDesign Problems and Solutions
Independent Groups Design Individual Differences
(Differences between subjects in each group) Use Repeated Measures Design to eliminate individual
differences (using same subjects)
Repeated Measures Design Differential Transfer
(Carryover effects) Use Independent Groups Design to eliminate differential
transfer
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Complex (Factorial) DesignsComplex (Factorial) Designs
Main Effects Effects of the Main IVs
Two possible Main Effects in your experiment Difference in RT between Caffeine and No Caffeine
(IV # 1 or “A”) Difference in RT of PH and NPH
(IV # 2 or “B”)
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Complex (Factorial) DesignsComplex (Factorial) Designs
Interaction Effects How one IV (A) may impact another IV (B)
Will Caffeine influence RT in one hand but not the other hand?
Four possible Interaction Effects in your experiment
This is a 2 X 2 Factorial Design 2 IVs (Caffeine & Handedness) Each has 2 levels (Caffeine or No Caffeine & PH or NPH)
Caffeine No Caffeine
PH RT (Caff&PH) RT (NoCaff&PH)
NPH RT (Caff&NPH) RT (NoCaff&NPH)
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Analysis of Factorial DesignsAnalysis of Factorial Designs
Analyze with a Factorial ANOVA (F test) F test analyses reflects
Systematic variance due to manipulation Error variance due to confounds
Including Individual differences of subjects F = variation between groups variation within groups
F = error variation + systematic variation error variation
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Analysis of Factorial DesignsAnalysis of Factorial Designs
F test analyses reflects
F = variation between groups variation within groups
F = error variation + systematic variation error variation
F test may indicate significant differences in Main Effects and Interaction Effects
Requires a Post Hoc text to determine differences
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Analysis of Factorial DesignsAnalysis of Factorial Designs
Post Hoc test for Main Effects One-Way or Repeated Measures ANOVAS if needed
Post Hoc test for Interaction Effects Graph data
Parallel lines indicate no interaction
Converging or Intersecting lines indicate interactionsCaffeine No Caffeine
0
200
400
600
PH
NPH
Caffeine No Caffeine0
100200300400500
PH
NPH
Caffein
e
No Caff
eine
0
200
400
PH
NPH
Caffein
e
No Caff
eine
0
200
400
PH
NPH
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Experimental DesignsExperimental Designs
EXPERIMENTAL DESIGNS THAT
ARE CORRECTLY
EXECUTED
RESULT IN SUCESSFUL OUTCOMES
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Experimental DesignsExperimental Designs
QUESTIONS?