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Chapter 9

Chapter 9. Producing Data: Experiments. Experimentation. Recall the distinction between experimental designs and observational designs In experimental studies, the investigator exposes individuals to a treatment to ascertain its effects. Vocabulary.

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Chapter 9

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  1. Chapter 9 Producing Data: Experiments Ch9: Experiments

  2. Experimentation • Recall the distinction between experimental designs and observational designs • In experimental studies, the investigator exposes individuals to a treatment to ascertain its effects Ch9: Experiments

  3. Vocabulary • Subjects = individuals participating in an experiment • Factors = specific experimental conditions or interventions applied to subjects • Treatment = a combination of a specific set of factors Ch9: Experiments

  4. Example: Effects of Advertising • Undergraduate students viewed a 40-minute video program that included ads for a digital camera • Two explanatory variables (factors): • Message length: 30-second vs. 90-second • Repetition: commercial shown 1, 3, or 5 times • Three response variables • recall of the ads after viewing • attitude toward the camera • intention to purchase Ch9: Experiments

  5. Treatment 3 = 30-second ad five times Illustrative Example: Treatments • Factor A: length of the commercial (2 levels) • Factor B: Number of repetitions (3 levels) • Thus: 2 × 3 = 6 treatments Ch9: Experiments

  6. Comparison Comparison is first principle of experimentation: The effects of a treatment can be judged only in relation to what would happen in its absence You cannot assess the effects of a treatment without a comparison group because: • Many factors contribute to a response • Conditions change on their own over time • The placebo effect and other passive intervention effects are operative Ch9: Experiments

  7. Randomization Randomization is the second principle of experimentation • Randomization = use of chance mechanisms to assign treatments • Randomization balances lurking variables among treatments groups, mitigating confounding by lurking variables! Ch9: Experiments

  8. Blinding Blinding is the third principle of experimentation • Blinding = assessment of the response in subjects is made without knowledge of which treatment they are receiving • Single blinding = subjects are unaware of treatment group • Double blinding = subjects and investigators are blinded Ch9: Experiments

  9. Illustrative Example: Quitting Smoking with Nicotine Patches • Explanatory variable: Nicotine patch / placebo patch • 60 subjects, 30 assigned to each treatment group • Response variable: Cessation of smoking (yes/no) • Design outline: Group 130 smokers Treatment 1 Nicotine Patch Random Assignment CompareCessation rates Treatment 2 Placebo Patch Group 230 smokers Source: JAMA, Feb. 23, 1994, pp. 595-600 Ch9: Experiments

  10. Randomizing Method • Number subjects 01,…,60 • Use table of random digits (TABLE B) • Select a line arbitrarily (e.g., line102)73|67|6 4|71|50| 99|40|0 0|19|27 • First four subjects are 50, 40, 19, and 27 • Keep using table until you get 30 subjects in Group 1 • The remaining subjects are assigned to Group 2 Ch9: Experiments

  11. Illustrative Example: Mozart, Relaxation and Performance on Spatial Tasks (Nature, 10/14/93, p. 611) • Subjects (30 undergraduate students) randomly assigned to one of three treatment groups • Group 1: Listen to Mozart • Group 2: Listen to relaxation tapes • Group 3: Silence • Response variable: change in IQ score Group 110 students Treatment 1 Mozart Random Assignment Group 210 students Treatment 2Relaxation CompareChange in IQ score Group 310 students Treatment 3Silence Ch9: Experiments

  12. The Logic of Randomization • Randomization encourages lurking variables to distribute evenly among treatment groups • Difference in the response at end of treatment are then due to either • Treatment or • Chance assignment of treatments • If the observed difference is larger than what would be expected just by chance, we say the results are statistically significant Ch9: Experiments

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