Third year ebm 2 2013

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Third year ebm 2 2013

  1. 1. Dr Ellie Hothersall This version: September 2013#DundeePublicH Evidence Based Medicine Part 2: Observational studies
  2. 2. Dr Ellie Hothersall This version: September 2013#DundeePublicH Evidence Based Medicine Part 2: Observational studies
  3. 3. Last week The world (of evidence based medicine) is very large… …We are very small Randomised controlled trials, grouped together into magnificent meta-analyses are the best way to obtain evidence that an intervention does or does not have an effect Twitter in lectures is interesting but complex (especially if not everyone can get wifi access) #DundeePublicH
  4. 4. #DundeePublicH
  5. 5. Twitter: a second attempt #Dundeepublich Switch off wifi on everything except one device – i.e. phone, OR laptop, OR ipad. #DundeePublicH
  6. 6. Today… AIM: To introduce some more key terms in Evidence Based Medicine and Research Methodology Objectives: Describe cohort and case control studies and describe the key requirements of both Define and describe Odds Ratio, Relative Risk, and Absolute Risk Be able to interpret Odds Ratio, Relative Risk and Absolute Risk #DundeePublicH
  7. 7. #DundeePublicH Learning Outcomes Describe the purpose of the following types of research study: Cohort study, Case control study Understand the types of outcome these studies are likely to examine and the type of evidence obtained Describe the key requirements and limitations of these types of studies Define, describe and interpret Odds Ratio, Relative Risk, and Absolute Risk
  8. 8. #DundeePublicH The Evidence Based Medicine Theme What? When? Research 1: The basics Neurology Research 2: More basics Neurology Research 3: Understanding p values Psychiatry Research 4: Critical appraisal Psychiatry Understanding Guidelines Ophthalmology Understanding Audit Reproduction
  9. 9. A question: #DundeePublicH
  10. 10. Why can’t we run a randomised controlled trial for everything? http://ndarc.med.unsw.edu.au/project/randomised-controlled-trial-rct-assess-acceptability-switch-buprenorphine-naloxone-suboxone%C2%AE #DundeePublicH
  11. 11. Money Do you know how much an RCT costs to run? http://www.moneysigns.net/british-money-bills/ #DundeePublicH
  12. 12. We already do things http://en.wikipedia.org/wiki/File:Illustration _Salix_alba0.jpg #DundeePublicH
  13. 13. Ethics http://www.flickr.com/photos/ooocha/ #DundeePublicH
  14. 14. http://health.howstuffworks.com/diseases-conditions/rare/10-rare-diseases.htm #DundeePublicH
  15. 15. #DundeePublicH
  16. 16. More ethics http://eofdreams.com/photo/poison/01/ What about interventions for which we might not get ethical approval? Or things out of our control? #DundeePublicH
  17. 17. Unintentional exposure http://fr.wikipedia.org/wiki/Bombardements_atomiques_d'Hiroshima_et_Nagasaki#DundeePublicH
  18. 18. Radiation exposure Survivors of Hiroshima and Nagasaki bombs in 1945 Data collected since 1950 Life span study (LSS) Mabuchi et al. http://www.irpa.net/irpa9/cdrom/VOL.1/V1_19.PDF#DundeePublicH
  19. 19. The structure of a cohort study http://sph.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/#DundeePublicH
  20. 20. The structure of a cohort study http://sph.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/#DundeePublicH
  21. 21. The structure of a cohort study http://sph.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/ http://freeonlinenews.o rg/weird-diseases/ http://eofdreams.com/death.html http://joeforamerica.com/2013/02/helping-friends-with-and-without-cancer #DundeePublicH
  22. 22. Types of cohort study Population Exposed Outcome No outcome Not exposed Outcome No outcome Prospective Retrospective #DundeePublicH
  23. 23. What do you get from a cohort study? Estimate of risk in the general population Ability to pick up infrequent occurrences Ability to find outcomes you hadn’t expected #DundeePublicH
  24. 24. #DundeePublicH
  25. 25. When is a general population not a general population? #DundeePublicH
  26. 26. If a condition is rare, what size group you need to detect it? http://www.theguvnorsassembly.com/forum/the-club/guvnor-for-sale/ #DundeePublicH
  27. 27. Rubbish in….? http://keywordingcentral.blogspot.co.uk/#DundeePublicH
  28. 28. But… Estimate of risk in the general population Prospective better for ensuring it really is “general” Ability to pick up infrequent occurrences Needs a large (= more expensive) group Ability to find outcomes you hadn’t expected Depends on you collecting data (prospective) or data quality (retrospective) #DundeePublicH
  29. 29. #DundeePublicH
  30. 30. #DundeePublicH
  31. 31. What does a cohort study tell us? Risk Disease status Present Absent Present Absent Population Exposed Outcome No outcome Not exposed Outcome No outcome a b c d #DundeePublicH
  32. 32. Risk Disease status Present Absent Present a b Absent c d #DundeePublicH
  33. 33. Relative risk (RR) Pevent when exposed RR= Pevent when not exposed a/a+b = d/c+d #DundeePublicH
  34. 34. Relative risk (RR) Pevent when exposed RR= Pevent when not exposed a/a+b = d/c+d Also known as a RISK RATIO Incidence in exposed Incidence in not exposed #DundeePublicH
  35. 35. Interpreting a Relative Risk Risk Ratio = 1#DundeePublicH
  36. 36. Interpreting a Relative Risk Risk Ratio <1 Logarithmic scale Incidence in exposed < incidence in control #DundeePublicH
  37. 37. Interpreting a Relative Risk Risk Ratio >1 Logarithmic scale Incidence in exposed > incidence in control #DundeePublicH
  38. 38. “If relative risk is the guy that drug companies always want to have at the party, absolute risk is the guy who never gets invited, the total buzz kill, the guy who showed up with someone's cousin once in a bad outfit and ended up mumbling to himself in the corner about how everything would be better if people just listened to him.” Heisel, 2010 http://www.reportingonhealth.org/blogs/next- pharma-love-fest-spend-some-time-absolute-risk #DundeePublicH
  39. 39. Which is sexier? 50% reduction in risk Reduction in risk from 1 in 200,000 to 1 in 400,000 #DundeePublicH
  40. 40. Which is sexier? 50% reduction in risk Reduction in risk from 1 in 200,000 to 1 in 400,000 #DundeePublicH
  41. 41. www.crestortouchpoints.com #DundeePublicH
  42. 42. www.crestortouchpoints.com #DundeePublicH
  43. 43. www.crestortouchpoints.com #DundeePublicH
  44. 44. www.crestortouchpoints.com #DundeePublicH
  45. 45. www.crestortouchpoints.com http://realestatebro wser.files.wordpres s.com/2012/04/wile- e-coyote-and-road- runner-hdforget-the- fiscal-cliff-cnbc- jpg_213601.jpg #DundeePublicH
  46. 46. P.S. Calculating Absolute Risk: Risk in exposed group – Risk in non exposed group #DundeePublicH
  47. 47. #DundeePublicH
  48. 48. Do mobile phones cause brain cancer? http://e-healthdiary.com/category/cancer/brain-cancer#DundeePublicH
  49. 49. How do you answer the question? Randomised controlled trial Cohort? #DundeePublicH
  50. 50. How a case control study works http://library.downstate.edu/EBM2/2500.htm#DundeePublicH
  51. 51. Case-control design Risk factor No risk factor Sample of CASES (with disease) Sample of Controls (disease-free) Risk factor No risk factor Exposure Disease Status Population #DundeePublicH
  52. 52. The direction of time Cases and controls identified now Data on past events collected Backwards in time Cases Data Controls http://www.yianniscove.com/film-of-the-day-back-to- the-future-1985-dir-robert-zemeckis/ #DundeePublicH
  53. 53. http://gocampussolutions.com/shop-2/suitcase-storage/ #DundeePublicH www.gocampussolutions.con
  54. 54. #DundeePublicH
  55. 55. http://www.wexphotographic.com/?/guides/exposure.html#DundeePublicH
  56. 56. #DundeePublicH
  57. 57. Do mobile phones cause brain cancer? http://e-healthdiary.com/category/cancer/brain-cancer#DundeePublicH
  58. 58. The INTERPHONE study Cardis et al, 2010. http://ije.oxfordjournals.org/content/39/3/675.full Regular use in the past ≥1 year Cases Controls Yes 1262 1174 No 1147 1488 #DundeePublicH
  59. 59. #DundeePublicH
  60. 60. What does a case control study tell us? Risk Disease status Present Absent Present Absent Population Exposed Outcome No outcome Not exposed Outcome No outcome a b c d #DundeePublicH
  61. 61. Risk Disease status Present Absent Present a b Absent c d #DundeePublicH
  62. 62. Odds Ratio (OR) Oddsevent when exposed OR= Oddsevent when not exposed a/c = b/d axd = bxc #DundeePublicH
  63. 63. Interpreting an Odds Ratio Odds Ratio = 1#DundeePublicH
  64. 64. Interpreting an Odds Ratio Odds Ratio <1 Logarithmic scale Risk in exposed < risk in control #DundeePublicH
  65. 65. Interpreting an Odds Ratio Odds Ratio >1 Logarithmic scale Risk in exposed > risk in control #DundeePublicH
  66. 66. Odds ratios are not odds! #DundeePublicH
  67. 67. Do mobile phones cause brain cancer? http://e-healthdiary.com/category/cancer/brain-cancer#DundeePublicH
  68. 68. The INTERPHONE study Cardis et al, 2010. http://ije.oxfordjournals.org/content/39/3/675.full Regular use in the past ≥1 year Cases Controls Yes 1262 1174 No 1147 1488 #DundeePublicH
  69. 69. Results Odds ratio for regular users compared with not-regular-users = 0.79 20% LESS likely #DundeePublicH
  70. 70. #DundeePublicH Makes sense?
  71. 71. Case-Control Studies Advantages Appropriate for rare diseases Can examine diseases with a long latency Can examine many potential risk factors Disadvantages No incidence / prevalence estimates Cannot measure disease onset Observer bias, Recall bias #DundeePublicH
  72. 72. Pitfalls Inappropriate controls Not from same source Bias in data collection Dependent on data from the past #DundeePublicH http://bygonebureau.com/2012 /04/13/instructions-for-viewing- the-bottomless-pit/
  73. 73. http://cisncancer.org/research/how_cancer_is_studied/epidemiological/study_types_03.html
  74. 74. Social acceptability bias http://www.mirror.co.uk/news/uk-news/investigation-pubs-underfilling-pints-of-beer-97368
  75. 75. Differences between Cohort and Case-control studies #DundeePublicH Cohort Case control Studies exposure to outcome Traces outcome back to exposure Can measure lots of outcomes Can measure lots of exposures Starts by identifying population Starts by identifying cases Relative risk Odds ratio Prone to bias from loss to follow up Prone to recall bias
  76. 76. Cohort study Study population Exposed Un-exposed Developed the outcome Did not develop the outcome Developed the outcome Did not develop the outcome Direction of the research inquiry Time #DundeePublicH
  77. 77. Case-Control study Study population With the Outcome Without the outcome Direction of the research inquiry Un-exposed Exposed Exposed Un-exposed #DundeePublicH Time
  78. 78. #DundeePublicH Research methods: the Evidence Pyramid http://www.mstrust.org .uk/professionals/infor mation/wayahead/articl es/16032012_08.jsp
  79. 79. Coming soon… #DundeePublicH http://www.ats.ucla.edu/stat/mult_pkg/faq/general/tail_tests.htm

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