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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

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Methods of experimental control

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Methods of experimental control l.jpg

Methods of experimental control

Randomization and Matching

Counterbalancing

Controlling participant and experimenter error


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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


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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.


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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


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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


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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.


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Latin squares

  • A B CA B C DA B C D

  • B C AB C D AB D A C

  • C A BC D A BC A D B

  • D A B CD C B A


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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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