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EXPERIMENTAL DESIGNS

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

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EXPERIMENTAL DESIGNS

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  1. EXPERIMENTAL DESIGNS • Criteria for Experiments • Independent, Dependent, and Confounding Variables • Types of Experimental Designs • Threats to Internal Validity • Threats to External Validity

  2. Criteria For Experimental Designs • Cause: experimenter manipulates a variable • Comparison: more than one condition • Control: extraneous variables are eliminated

  3. Independent Variable • variable manipulated by the experimenter • levels • conditions

  4. Dependent Variable • variable measured to assess the effect of the independent variable

  5. Confounding Variable • variable other than IV and DV which changes between conditions • control variable: potential confounding variable that is controlled

  6. Types of Experiments • between subjects • matched groups • within subjects

  7. Threats to Internal Validity:Individual Differences • Systematic differences between individuals in different groups • Strategies • random assignment • matched groups • within subjects design

  8. 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

  9. 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

  10. Instrumentation • Changes in the measuring instrument or equipment • Strategy • Use standardized administration

  11. 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

  12. 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

  13. Demand Characteristics • Cues from the experimenter or research procedure about what behavior is desired • Strategy • Single-blind procedure

  14. Experimenter Effects • Experimenter’s expectations affect measurements • Strategy • Double-blind procedure

  15. Floor and Ceiling Effects • Measuring instrument is not sensitive enough • Floor effects • Ceiling effects • Strategy • Check sensitivity of instrument prior to experiment

  16. 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

  17. Order Effects • Also called Testing or Repeated Testing • Effects of repeated measurements • Fatigue effects • Practice effects • Carryover effects • Strategy • Counterbalance order of conditions

  18. 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

  19. 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

  20. Latin Square Counterbalancing • Each condition is presented in each position for an equal number of subjects • Controls for practice and fatigue effects

  21. 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

  22. 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)

  23. 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

  24. Randomized Counterbalancing • Used when there are multiple stimuli tested for each condition • Put the stimuli in random order for each participant

  25. Threats to External Validity • Unrepresentative Sample • use random or stratified random sampling • do exact or systematic replications • Artificiality • use a more realistic setting • do systematic or conceptual replications

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