1. Stat 512 Day 2: Designing Experiments

2. Leftovers from Tuesday Questions on syllabus? Repeating the question Blackboard problems?  Sooner turn in, sooner get feedback?

3. Last Time Microwave popcorn factory  Two categorical variables level of exposure, whether airway obstruction  Graphical summary: segmented bar graph  Numerical summary: difference in conditional proportions  Not able to draw cause-and-effect conclusions Other differences between the two groups that might explain the higher airway obstruction rates in high exposure group?  Not able to generalize to all microwave plant workers “Healthy worker effect”

4. Last Time Smoking and lung cancer  Two categorical variables (amount of smoking and with or without disease)  Graphical summary: segmented bar graph  Numerical summary: difference in conditional proportions  Can’t draw cause and effect conclusion Could be some other difference (diet) between the EV groups that explains higher lung cancer rates with more smoking

5. Last Time Generalizing from sample to population?  Reasonable to conclude 605/(605+780), or 44%, of all males of similar ages and economic status have lung cancer? Measurement issues  Relying on recall…  Know they are sick…

6. Second famous smoking study Hammond and Horn (1958) Find 12,000 healthy men, complete a questionnaire on smoking habits, had 22,000 American Cancer Society volunteers follow them for 44 months to see whether they die from lung cancer  Advantages? Disadvantages?

7. Practice Problems Identifying variables  Supreme Court Justices: Qualitative: gender, party Quantitative: age, number of yes votes  Not OK: number of republicans

8. Practice Problems Victims of violence  OU: people (not “number of victims”)  EV: whether abused  RV: whether commit crime  Not ok in defining variables: Those who… Number abused… Whether abuse leads to violent crime…  (c) Mostly to spur discussion… Which variable was “controlled” by the researchers…

9. More practice: The book Day Hikes in San Luis Obispo County by Robert Stone gives information on 72 different hikes that one can take in the county. For each of the 72 hikes, Stone reports the distance of the hike (in miles), the anticipated hiking time (in minutes), the elevation gain (in feet), and the region of the county in which the hike can be found (North County, South County, Morro Bay, and so on, for a total of eight regions).  Observational units?  Types of variables

10. More Practice: Hiking in SLO Are these legitimate variables?  Longest hike in the book  Those hikes in the North County region  Proportion of hikes with an elevation gain of more than 500 feet  Is hiking time related to elevation gain?

11. Example 1: Near-sightedness and Night Lights “Myopia and ambient lighting at night,”  Quinn, G.E., Shin, C.H., Maguire, M.G. and Stone, R.A.  Nature, 399:113-114, 1999.

12. Example 1: Myopia and Night Lights Room lightNight lightDarkness Far-sighted123940 Normal22115114 Near-sighted417818

13. Example 1: Myopia and Night Lights Base rate:.286 Conditional Proportions  Room light:.55 myopia,.16 hyperopia  Night light:.336 myopia,.168 hyperopia  Darkness:.105 myopia,.232 hyperopia Convincing evidence that using more light in the child’s room causes a higher rate of myopia?

14. Terminology: Confounding variable Has an influence on the response, but its effects cannot be separated from those of the explanatory variable room light parents with bad eyes darkness parents with good eyes Compare eye-sight

15. Example 2: Have a Nice Trip Can instruction in a recovery strategy improve an older person’s ability to recover from a loss of balance?instruction  12 subjects have agreed to participate in the study  Assign 6 people to use the lowering strategy and 6 people to use the elevating strategylowering  Similar amounts of men in both groups? Proportion of group 1 that are male – proportion of group 2 that are male What would we like to be true about these proportions?

16. Investigation 1-7: Have a Nice Trip Take 12 index cards Put one of the 12 names on each card Shuffle the cards and deal out 6 to learn the lowering strategy and the other 6 to learn the elevating strategy What is the number of males in each group? The male proportion in each (out of 6)? What is the difference in these 2 proportions?

17. Pool Results Quantitative variable Different type of graph…  What are the observational units and variable in this graph?

18. Long-term pattern? Use applet to repeat the process a large number of times Open IE, double click on “Dr. Beth Chance” > “Statistical Methods” > “Stat 512 Java Applets” > “Randomizing Subjects The applet mimics exactly what you did with the index cards.

19. Effect of randomization If you randomly assign subjects to the groups, what is generally true about the groups? If after imposing the treatment, you later observe a difference between the groups, to what can you attribute that difference?

20. Moral Randomization equalizes variables between groups  Should not have potentially confounding variables  If later (after treatments) observe a difference between groups, feel comfortable attributing that difference to the explanatory variable Remaining question  Will always be some difference, by chance  How big does this difference have to be?

21. Example: Friendly Observers “The trouble with friendly faces: Skilled performance with a supportive audience,”  Butler, J. L., and Baumeister, R. F.  Journal of Personality and Social Psychology, 75: 1213-1230, (1998).

22. To do By Friday, noon  Turn in HW 1 (4 problems) By Tuesday  Preview the Friendly Observers example (complete questions (a)-(e))  Submit PP 2 in Blackboard  Pick and start reading one of the 3 articles