Methods of experimental control

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Methods of experimental control - PowerPoint PPT Presentation

Methods of experimental control. Randomization and Matching Counterbalancing Controlling participant and experimenter error. Randomization. Random selection: a representative sample Basis of generalizability or external validity Exhaustive sampling is good, but difficult

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Methods of experimental control

Randomization and Matching

Counterbalancing

Controlling participant and experimenter error

Randomization
• Random selection: a representative sample
• Basis of generalizability or external validity
• Exhaustive sampling is good, but difficult
• Random assignment: equivalent groups
• Basis of controlling most evars
• Chance of nonequivalent groups covered in error term of statistical analysis
• Random sequencing: order effects
Matching
• Since randomization assigns individual differences to the error term, it has one unfortunate effect: reducing power.
• Matching techniques increase group equivalence, so it increases power
• Matching must be complemented with random assignment to produce randomized blocks designs or matched groups designs.
Matching tactics
• Hold extraneous variable constant
• Limits population size
• Limits generality of results
• Manipulate extraneous variable in design
• Beware whynot variables
• Use a yoked control design
• Equate participants: Precision control
• Equate participants: Frequency distribution
Counterbalancing
• Sequencing effects:
• Order effect
• Carry-over effect
• Counterbalancing tactics
• Intrasubject for repeated measures: ABBA
• Intrasubject/intragroup: ABBA/BAAB
• Intragroup
• Incomplete: Latin and Greco-Latin squares
Latin Squares
• In Latin Square counterbalancing, each different treatment is symbolized by a letter, different applications of treatments are in columns, and different occasions are in rows.
• The number of rows and columns equals the number of treatments.
• Each treatment letter appears once and only once in each row and in each column.
Latin squares
• A B C A B C D A B C D
• B C A B C D A B D A C
• C A B C D A B C A D B
• D A B C D C B A
Controlling experimenter and participant effects
• Double-blind placebo model
• Disguise techniques
• Retrospective report, probes, and the think-aloud technique: Reactive effects caveat.
• Caution
• Blind techiques
• Automation