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Chapter 9 – Lecture 9 Control to Reduce Threats to Validity

Chapter 9 – Lecture 9 Control to Reduce Threats to Validity. How to control threats. General control procedures Control over subject and experimenter effects Control through participant selection and assignment Control through specific experimental design. “Control For”.

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Chapter 9 – Lecture 9 Control to Reduce Threats to Validity

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  1. Chapter 9 – Lecture 9 Control to Reduce Threats to Validity

  2. How to control threats • General control procedures • Control over subject and experimenter effects • Control through participant selection and assignment • Control through specific experimental design “Control For”

  3. 1. General Control Procedures • Preparation of Setting – LAB = CONTROL Control over research setting Lab ++internal validity --generalization Applied setting --internal validity ++generalization LAB = tradeoff with external validity??? Natural environment In the lab Stanford Prison Experiment • the psychological effects of becoming a prisoner or prison guard - set up a simulated a prison

  4. Planned simulation of 2 weeks (1971) N = 24 college students from the U.S. $15/day by participating in a study Flip of coin prisoner & guard basement of Stanford's Psychology Department building

  5. 1. General control procedures – threats to validity a. Preparation of setting – LAB = CONTROL

  6. 1. General control procedures – threats to validity b. Response Measurement Focus on the DV: careful selection of the measurement • scale…reliable & validated (scale development) • equipment • techniques c. Replication - Pilot experiment: dress rehearsal • Exact replication: ditto • Systematic replication: modify theory • Conceptual replication: repeat with different OD of variables

  7. 2. Control over Subject & Experimenter effect - motivation - knowledge - expectations • blind • double blind procedures Experimenter knows the hypothesis, subject assignment etc… Blind: participant doesnot Know or is “blind” to assignment into a group Double Blind: both RA & Subject do not know (drug studies/ light therapy)

  8. 3. Control through subject selection & assignment • Random Sampling!!! Everyone has = chance for selection only way to control for unknown factors (potential confounds) Potential confounds are evenly distributed! • Subject Assignment – random assignment!!! free random assignment (use random number table) matched random assignment – (age, weight) 1 2 Unbiased sample Table of random # or r# generator 3 General Population Everyone.. Target population Representative sample (n)

  9. 4. Control over experimenter design  Simple Pretest-posttest design  Pretest-posttest, control group design

  10. Control through Design Single group Pretest-posttest Pretest Group A TX Posttest Compare Randomized, Pretest-Posttest, Control Group Design R Group A Pretest TX Posttest (Ex) R Group B Pretest no TX Posttest (Con) Compare

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