Backpacks and Back Pain

1. Backpacks and Back Pain Is there an association between the hypothesized cause and the disease?

2. Hypotheses 1 How is this disease distributed? 2 Is there an association? 3 Is the association causal? 4 What should be done to prevent the disease? 5 Did the prevention strategy work? Where are we and where are we going? What’s my hypothesis? Did the exposure and the disease turn up together?

3. Review Under what circumstances is it appropriate to experiment with human beings? How can we test a hypothesis that something causes harm in a human experiment? How can we test the hypothesis that carrying a heavy backpack causes back pain in a way that follows the principles you developed in the previous investigation?

4. Carrying Heavy Backpacks Causes Back Pain.

5. Epi Talk Epi Talk Natural Experiment Naturally occurring circumstances in which groups of people within a population have been exposed to different levels of the hypothesized cause of an outcome.

6. Epi Talk Epi Talk Observational Studies Epidemiologic studies of natural experiments in which the investigator is not involved in the intervention other than to record, classify, count, and statistically analyze results.

7. Epi Talk Epi Talk Trial An epidemiologic experiment in which subjects are assigned into groups to receive or not receive a hypothesized beneficial intervention.

8. Epi Talk Epi Talk Experimental Group Subjects selected for a trial who are assigned to a group to receive a hypothesized beneficial intervention.

9. Smoke Do Not Smoke Trial Hypothesis: Cigarette smoking causes lung cancer.

10. Smoke Do Not Smoke Trial Hypothesis: Cigarette smoking causes lung cancer.

11. Testing Hypotheses Ache!

12. Testing Hypotheses Control Group Relative Risk Inferences 2 X 2 Table Risks Belmont Report Carrying heavy backpacks causes back pain.

13. Trial No Acne Acne Total AcneMed 100 a b c d No AcneMed 100 Carrying heavy backpacks causes back pain.

14. No Back Pain Back Pain Heavy Backpacks a b c d No Heavy Backpacks Observational Study Total Carrying heavy backpacks causes back pain.

15. ? 100 ? 100 There are no 100’s in the “Total” column. (No “director” saying who will have a heavy or light backpack!) Testing Hypotheses No Back Pain No Acne Back Pain Total Acne Total Heavy Backpack AcneMed 100 a b a b c d c d No Heavy Backpack No AcneMed 100 Observational Study Trial What is the difference?

16. “Heavy” Carry No More Than 15% of Your Body Weight

17. 120 lbs 15 % What is 15% of someone’s body weight? What is 15% of 120 lbs? 18 lbs 120 X .15 =

18. 20 lbs Heavy 15% of Maria’s body weight = 18 lbs Is Maria’s backpack heavy? Yes

19. Epi Log Worksheet

20. 1 Exposure Check one. Heavy Backpack No Heavy Backpack

21. Outcome What is back pain? Any memorable pain in the past two weeks.

22. 2 Outcome Check one Back Pain No Back Pain

23. No Back Pain Back Pain Heavy Backpacks No Heavy Backpacks 2 x 2 Table Total a b c d Can you think of someone, anyone, anywhere in the whole wide world, who could not “fit” into cell a, b, c, or d?

24. Investigation 2-4

25. Epi Talk Epi Talk Informed Consent Voluntary consent given by a person for participation in a study. Participants must know and understand the study, give consent without coercion, and know that they can withdraw at any time.

26. Investigation 2-4

27. No Back Pain Back Pain Heavy Backpacks No Heavy Backpacks 2 x 2 Table Total      a b c d    

28. 3 2x2 Table Back Pain No Back Pain Total Heavy Backpacks a b c d No Heavy Backpacks

29. 4 Risk Back Pain Risks No Back Pain Back Pain Total Heavy Backpacks % or a b c d No Heavy Backpacks % or Numerator is the number of people who have back pain and denominator is the number of people in the population.

30. 5 Relative Risk Back Pain Risks Relative Risk Back Pain No Back Pain Total Heavy Backpacks % or a b c d No Heavy Backpacks % or Divide the risk of the outcome in the exposed group by the risk of the outcome in the unexposed group.

31. 6 Relative Risk Back Pain Risks Relative Risk Back Pain No Back Pain Total Heavy Backpacks % or a b c d No Heavy Backpacks % or Students who carried heavy backpacks were ______ times as likely to have back pain as students who did not carry heavy backpacks.

32. 7 Inference Back Pain Risks Relative Risk Back Pain No Back Pain Total Heavy Backpacks % or a b c d No Heavy Backpacks % or Predict from what is observed to what is not observed.

33. Carrying Heavy Backpacks Causes Back Pain Trial Heavy Backpacks No Heavy Backpacks

34. Carrying Heavy Backpacks Causes Back Pain Observational Study Heavy Backpacks No Heavy Backpacks

35. Carrying Heavy Backpacks Causes Back Pain How could the hypothesis be tested in a trial? What “principle” would be broken in a trial? Was the same “principle” broken in the observational study?

36. 8 Review Were you involved in assigning exposures? Epi Talk Observational Studies Epidemiologic studies of natural experiments in which the investigator is not involved in the intervention other than to record, classify, count, and statistically analyze results. Did you observe what others had done to themselves? Did you record, classify, count and statistically analyze the results?

37. Carrying Heavy Backpacks Causes Back Pain. Trial Heavy Backpacks No Heavy Backpacks

38. Carrying Heavy Backpacks Causes Back Pain. Observational Study Heavy Backpacks No Heavy Backpacks

39. Trial Carrying Heavy Backpacks Causes Back Pain Heavy Backpacks No Heavy Backpacks

40. Carrying Heavy Backpacks Causes Back Pain We are all carrying out “natural experiments” on ourselves each day of our lives, as we decide what to do, where to go, and to what to expose ourselves.

41. CDC Backpacks and Back Pain Investigation 2-4 has ended.