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EXPERIMENTAL DESIGN.  Three conditions  Covariation  Time-order relation  Elimination of plausible alternatives.

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Presentation on theme: "EXPERIMENTAL DESIGN.  Three conditions  Covariation  Time-order relation  Elimination of plausible alternatives."— Presentation transcript:


2  Three conditions  Covariation  Time-order relation  Elimination of plausible alternatives

3  Loftus and Burns (1982) Conducted 3 experiments in which Ss saw either a mentally shocking event (in which a young boy was shot in the face) or a nonviolent version of the same film. In Exp I, 226 university students viewed 1 version of the film and answered questions on its contents and their own feelings. In Exp II, 180 college students viewed a version of the film or another nonviolent film and answered a forced-choice questionnaire. In Exp III, 160 Ss viewed either film or a 3rd version with a surprise, nonviolent ending and then answered a questionnaire. Compared to Ss who saw a nonviolent version of the same film, those who saw the mentally shocking version showed poorer retention of the details of the film. Retention was poorer whether measured by recognition or recall. Furthermore, impaired memory occurred only when the event was mentally upsetting and not when it was merely unexpected but not upsetting. Results suggest that mentally shocking episodes may disrupt the lingering processing necessary for full storage of information in memory. (26 ref) (PsycINFO Database Record (c) 2007 APA, all rights reserved)

4  Covariation- 4% recall in violent - 28% recall in non-violent  Time-order – differences AFTER IV  Alternative explanations?  Recall versus recognition  Control for surprise  Additional controls  Controlled variables? (instructions, film, etc)  Balanced variables? (memory of P, attentiveness, etc)

5  Between subjects (Independent Groups)  each group receives level of IV  each person participates in experiment once  E.g. Brady (1959) “Executive Monkey” experiment  Contrast with Seligman’s “Learned Helplessness”  Illustrates biggest potential confound in Between design

6  “Control” of a priori group differences  Randomization  Matching  Within subjects (Repeated Measures)  Subjects receive all levels of IV  Each participant serves as own control  E.g. Jarrad (1963) “Stoned rats experiment”  LSD and food reward  6 levels of IV  Biggest potential confound with within-subjects  Ways to manage CARRYOVER EFFECTS


8  Decisions, decisions, decisions…….  What is the question?  Research questions and hypotheses  Operational definitions  The Independent Variable  Types of manipulations  Physiological  Experience  Stimulus or Environment  Participant Characteristics (STOP!!!)

9  Are you a good IV or a bad IV?  Internal validity  Extraneous variables and Confounding  The Dependent Variable  Types of Measurements  Response rate or Frequency of response (occurrence)  Degree or Amount (quantity)  Latency or duration (time)  Are you a good DV or a bad DV?  Validity  Reliability  Selecting Participants  How to get ‘em or choose ‘em  Precedent  Availability  Appropriateness

10  How many of ‘em do I need?  Is more always better?  Statistical Power  Hypothesis testing and errors  Type I and Type 2

11  A graphic example of power True mean difference = 8 True mean difference = 16

12  Claiming your power  Effect size  Sample size  Decrease SD  Tolerate alpha  Use a one tailed test  Use a within-subjects design  Independent means and Repeated Measures t

13  When absolute power corrupts absolutely……  Statistical versus Practical significance  Significance tests and tests of Effect Size  Finding a reasonable compromise CORRUPTION for $100 Italian political philosopher who’s name has (fairly or not) become synonymous with unscrupulous cunning, dishonesty, deception or expedience. Who is Niccolo Machiavelli? I’ll take CORRUPTION for $200, Alex….. British historian of the late 19 th century who is credited with the quote above. Who is Lord Acton?

14  Keep an eye out for these dangers…..  Perceptions matter ….. YOU!!!  Experimenter as confounding variable  Gender, age, attractiveness, and many things you never thought about!  Kim and her rats  Experimenter as un-objective observer  Effects of expectancies  Rosenthal effect – the dumbing down of rats….  Automate experimenter (instructions, protocol)  Automate measurement  Single blind and double blind experiments

15  Perceptions matter…. THEM!!!  Demand characteristics  “Good” subjects  Social desirability  Experimenter pleasers  “FU” subjects  Other sources of participant bias  Response biases  Experimental effects  Response patterns (yea and nay-sayers)

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