1. 1-1 Design Unit 1 Study Designs For Data Collection Three basic study designs 1. Controlled Experiments (Chapt. 1) 2. Observational studies (Chapt. 2) 3. Sample Surveys (Chapt. 19 + supplements) A.05

2. 1-2 Design 1. Experimenter gives "treatment" to "treatment group". 2. Experimenter does NOT decide who is in the "treatment group. 3. Type of observational study.We look at a SAMPLE of individuals and infer what their population is like.

3. 1-3 Design Some Definitions Study units, Experimental units: "subjects". Population: The set of individuals of interest. Sample: Subset of the population. Variable: Characteristic or property of an individual population unit.

4. 1-4 Design Some More Definitions Response: Variable of interest in an experiment or other study. What is observed. Factors: Those variables whose effect on the response is of interest Treatment factors: What we want to assess Confounding factors: Factors whos effects might be confounded (mixed up) with the effects of the treatment Want to avoid, if possible. Treatment: The thing whose effect is being investigated. Treatment group: The group of subjects to which the treatment is applied. Blind and Double-blind studies

5. 1-5 Design Salk Vaccine Trials Testing Drug Effectiveness Starting in 1916 polio claimed hundreds of thousands of lives, many children. In 1954, Jonas Salk’s vaccine seemed promising. National Foundation for Infantile Paralysis were ready to test the vaccine. Vaccinated: about 1/2 million children Unvaccinated: about a million children Refused Vaccination: about 1/2 million children

6. 1-6 Design Some Basic Principals We must be able to make COMPARISONS. Treatment versus Control - The treatment and control groups should be as similar as possible - except for the treatment. - If they differ by some other unknown factor (income, age, exposure, …) then the effect of this other factor is confounded with the treatment effect. Randomized controlled study - Use random assignment of eligible subjects to treatment or control group. (Do not use human judgement.) Placebo - For example, children in the control group received a saline solution. Double-blind assessment - Neither subjects nor physicians know the group assignment.

7. 1-7 Design Designed Experiments example: "Assessing the Impact of Short-Term Supermarket Strategy Variables", Wilkinson, Mason & Paksoy, J. of Marketing Research, Feb.1982, pp72-86. Factors: Factor Levels: Response: Experimental Units: Location: Design:

8. 1-8 Design Piggly-Wiggly Want to examine the importance / effect of (Factors) - to (Response)

9. 1-9 Design Regular Price Price LevelCost Price Reduced Price Normal Display Display Level Expanded Display Special Display Advertising LevelAdvertising No Advertising Factorial Design

10. 1-10 Design Difficulties in Conducting a Study 1. Time 2. Cost 3. Unrealistic setting (To what population is the result applicable?) 4. Ethics

11. 1-11 Design Designed Experiment versus Observational Study In a DESIGNED EXPERIMENT the investigator assigns experimental units to treatments. versus In an OBSERVATIONAL STUDY the investigator does not assign experimental units to treatments, but merely observes the treatments and the response on a set of experimental units.

12. 1-12 Design Observational Studies Always find out IN DETAIL how the controls were selected! Watch out for confounding factors! Often one can "control for confounding factors": Make comparisons in smaller, more homogeneous subgroups, rather than over the whole. Example: "Sex Bias in Graduate Admissions: Data from Berkeley"