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HS 67Ch9: Experiments1 Chapter 9 Producing Data: Experiments

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HS 67Ch9: Experiments2 Experimentation Recall the distinction between experimental designs and observational designs In experimental studies, the investigator exposes individuals to a treatment to ascertain its effects

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HS 67Ch9: Experiments3 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

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HS 67Ch9: Experiments4 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

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HS 67Ch9: Experiments5 Illustrative Example: Treatments Factor A: length of the commercial (2 levels) Factor B: Number of repetitions (3 levels) treatmentsThus: 2 × 3 = 6 treatments Factor B: Repetitions 1 time3 times5 times Factor A: Length 30 seconds seconds 456 Treatment 3 = 30-second ad five times

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HS 67Ch9: Experiments6 Comparison 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 Comparison is first principle of experimentation: The effects of a treatment can be judged only in relation to what would happen in its absence

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HS 67Ch9: Experiments7 Randomization Randomization = use of chance mechanisms to assign treatments Randomization balances lurking variables among treatments groups, mitigating confounding by lurking variables! Randomization is the second principle of experimentation

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HS 67Ch9: Experiments8 Blinding 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 Blinding is the third principle of experimentation

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HS 67Ch9: Experiments9 Illustrative Example: Quitting Smoking with Nicotine Patches u Explanatory variable: Nicotine patch / placebo patch u 60 subjects, 30 assigned to each treatment group u Response variable: Cessation of smoking (yes/no) u Design outline: Source: JAMA, Feb. 23, 1994, pp Random Assignment Group 1 30 smokers Treatment 1 Nicotine Patch Compare Cessation rates Group 2 30 smokers Treatment 2 Placebo Patch

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HS 67Ch9: Experiments10 Randomizing Method Number subjects 01,…,60 Use table of random digits (TABLE B) Select a line arbitrarily (e.g., line102) 73|67|64|71|50|99|40|00|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

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HS 67Ch9: Experiments11 Illustrative Example: Mozart, Relaxation and Performance on Spatial Tasks (Nature, 10/14/93, p. 611) u Subjects (30 undergraduate students) randomly assigned to one of three treatment groups u Group 1: Listen to Mozart u Group 2: Listen to relaxation tapes u Group 3: Silence u Response variable: change in IQ score Random Assignment Group 1 10 students Treatment 1 Mozart Compare Change in IQ score Group 3 10 students Treatment 3 Silence Group 2 10 students Treatment 2 Relaxation

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HS 67Ch9: Experiments12 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

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