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Methods of experimental control

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

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  1. Methods of experimental control Randomization and Matching Counterbalancing Controlling participant and experimenter error

  2. 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

  3. 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.

  4. 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

  5. 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

  6. 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.

  7. 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

  8. 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

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