DAY 4 Considerations for Interpretation Confounding

1. DAY 4 Considerations for Interpretation Confounding

2. Introduction Understanding what the data can tell us and cannot tell us

3. Approach to understanding the data If we see an association, what other possible explanations can there be besides causation? - Confounding

4. Confound “To mistake for another” “To confuse” Webster’s Online Dictionary www.webster-dictionary.org

5. Guilt by association

6. Guilt by association George Not Observed Observed Broken Window X James Cause

7. Scenario 1 – Individual level Hugo Lennie

8. Association Cause Scenario 1 – Individual level Not Observed Hugo Lennie Observed X Empty Beer Bottles Cause

9. Scenario 2 – Population level

10. Yes R-Rated Movies No R-Rated Movies Scenario 2 – Population level Prevalence Ratio Not Try Alcohol Prevalence Try Alcohol a b 6 c d 90 People who _____________ are ______times as likely to __________ as people who ____________.

11. Cause Scenario 2 – Population level “If we can keep our teenagers from watching R-rated movies, then we can go a long way towards preventing teenagers from trying alcohol.” “The study establishes a clear progression that begins with watching R-rated movies and leads to teenagers trying alcohol.” Watching R-Rated Movies Trying Alcohol

12. Association Cause X Watching R-Rated Movies Trying Alcohol Cause Scenario 2 – Population level ?

13. Confounding A situation in which an association between an exposure and an outcome is observed as a result of the influence of a third, unobserved, factor, called a confounder. The true effect of an exposure on an outcome is distorted because it is mixed up with another factor that can influence the outcome. The confounder is both associated with the observed exposure and is a possible cause of the outcome.

14. Unobserved Exposure Watch out for lurking variables! Limited Adult Supervision Watch R-rated movies Try alcohol

15. Unobserved Exposure Willingness to Break the Rules Watch R-rated movies Try alcohol

16. Unobserved Exposure Poor Self-Control Watch R-rated movies Try alcohol

17. Unobserved Exposure Parental attitudes Watch R-rated movies Try alcohol

18. Unobserved Exposure Risk taker Watch R-rated movies Try alcohol

19. Lack of Supervision or Poor Self- Control or Risk taking or . . . X … and you avoided or eliminated the hypothesized cause, what would happen to the outcome? If the association was due to confounding … ? Watch R-rated movies Try alcohol

20. Association Cause X Cause Playing Volleyball Skin Cancer Confounder

21. Association Cause X Cause Drinking Alcohol Lung Cancer Confounder

22. Association Cause X Cause Eating Pretzels Auto Accidents Confounder

23. Association Cause X Cause Watching TV Acne Confounder

24. Association Cause X Cause Drinking Coffee Cancer Confounder

25. Association Cause X Cause Driving Motorcycles Hepatitis C Confounder

26. Association Cause X Cause Playing Baseball Oral Cancer Confounder

27. Association Cause X Cause Skin Cancer Sand Confounder

28. Always Remember Association is not necessarily causation. Association is not necessarily causation. Association is not necessarily causation. Association is not necessarily causation. Association is not necessarily causation. Association is not necessarily causation. Association is not necessarily causation. Association is not necessarily causation.

29. Supplementary Lesson about Confounding Confounding: Working the Numbers

30. Possible Confounder Two criteria for Confounding • The confounder is associated with the observed exposure (but not affected by the exposure). • The confounder is a possible cause of the outcome. Exposure Outcome

31. Possible Confounder • We suspect a factor to be a potential confounder based on intuition and evidence that it is associated with the exposure under study and a cause of the outcome under study • But is this true in the data at hand? • We can assess this by calculating the strength of association between: 1) exposure and the suspected confounder; and 2) between suspected confounder and outcome. PR = ? PR = ? Exposure Outcome PR = ?

32. People who exercise less are more likely to have high blood pressure. No increased BP Increased BP Prevalence Ratio Prevalence Total 400 Exercise less 40 % 400 600 1000 1000 a b c d 2.0 Exercise more 200 20 % 200 800 1000 1000 2000 a a+b c c + d ÷ People who exercise less are 2.0 times as likely to have increased blood pressure as those who exercise more.

33. Association Cause X Cause Exercise less Increased blood pressure Confounder?

34. Smoking Assessment of smoking as a potential confounder See if it meets two criteria for confounding 2. A possible cause of the outcome? 1. Associated with the exposure? PR = ? PR = ? Exercise Less Increased blood pressure PR = 2.0

35. 1. Is the potential confounder associated with the exposure? (Is smoking associated with exercise?) Non-smokers Prevalence Ratio Smokers Prevalence Total 450 Less exercise 45.0 % 450 550 1000 1000 a b c d 2.6 More exercise 175 17.5 % 175 825 1000 1000 2000 People who exercise less are 2.6 times as likely to be smokers as those who exercise more.

36. Smoking PR = ? PR = 2.6 Exercise Less Increased Blood Pressure PR = 2.0

37. 2. Is the potential confounder a possible cause of the outcome? (Is smoking a possible cause of increased blood pressure?) No increased BP Increased BP Prevalence Ratio Prevalence Total 475 Smokers 76.0 % 475 150 625 625 a b c d 7.6 125 Non-smokers 10.0 % 125 1250 1375 1250 2000 Smokers are 7.6 times as likely to have increased blood pressure compared to non-smokers.

38. Smoking Is there Possible Confounding? PR = 7.6 PR = 2.6 Exercise Less Increased Blood Pressure PR = 2.0

39. Marijuana Use and Getting Tattoos

40. People who smoke marijuana are more likely to get tattoos than people who do not smoke marijuana. No Tattoos Prevalence Ratio Tattoos Prevalence Total 50 MarijuanaUser 25 % 50 150 200 200 a b c d 1.7 Not a marijuana User 30 15 % 30 170 200 200 a a+b c c + d ÷ People who ____________________________________________ are ______ times as likely to _______________________________ compared to people who __________________________________

41. People who smoke marijuana are more likely to get tattoos than people who do not smoke marijuana. No Tattoos Prevalence Ratio Tattoos Prevalence Total 50 MarijuanaUser 25 % 50 150 200 200 a b c d 1.7 Not a marijuana User 30 15 % 30 170 200 200 People who use marijuana are 1.7 times as likely to get tattooed compared to people who do not use marijuana.

42. Association Cause X Cause Use Marijuana Get Tattoos Confounder?

43. Access to Money Assessment of “access to money” as a potential confounder See if it meets two criteria for confounding 2. A possible cause of the outcome? 1. Associated with the exposure? PR = ? PR = ? Use Marijuana Get Tattoos PR = 1.7

44. 1. Is the potential confounder associated with the exposure? (Is access to money associated with marijuana use?) No Access to Money Access to Money Prevalence Ratio Prevalence Total 150 MarijuanaUser 75.0 % 150 50 200 200 a b c d 3.0 Not a marijuana User 50 25.0 % 50 150 200 200 400 People who use marijuana are 3.0 times as likely to have access to money than non-users.

45. Access to Money PR = ? PR = 3.0 Use Marijuana Get Tattoos PR = 1.7

46. 2. Is the potential confounder a possible cause of the outcome? (Is access to money a possible cause of getting tattoos? No Tattoos Prevalence Ratio Tattoos Prevalence Total 60 Access to Money 60 140 200 42.8% 140 a b c d 3.9 No Access to Money 20 11.1 % 20 180 200 180 400 People with access to money are 3.9 times as likely to get tattoos than people without access to money.

47. Access to Money Is there Possible Confounding? PR = 3.9 PR = 3.0 Use Marijuana Get Tattoos PR = 1.7

48. Possible Confounder Another Example - Is there Possible Confounding? A possible cause of the outcome? Associated with the exposure? PR = 1.10 PR = 0.97 Exposure Outcome PR = 1.8

49. Day 4 First Morning Breakout Assessing Possibility of Confounding

50. Deck Worksheet – pages 3-5 Use the 54 cards in the deck to explore the potential for confounding in this data set