Third year ebm 2 2013
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This document provides an overview of observational studies in evidence-based medicine, including cohort and case control studies. It defines cohort and case control studies, describes their key requirements and limitations. It also defines and describes odds ratio, relative risk, and absolute risk, and how to interpret these measures of effect. Examples are provided to illustrate mobile phone use and risk of brain cancer from the INTERPHONE study.
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Third year ebm 2 2013
- 1. Dr Ellie Hothersall This version: September 2013#DundeePublicH Evidence Based Medicine Part 2: Observational studies
- 2. Dr Ellie Hothersall This version: September 2013#DundeePublicH Evidence Based Medicine Part 2: Observational studies
- 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
- 5. Twitter: a second attempt #Dundeepublich Switch off wifi on everything except one device – i.e. phone, OR laptop, OR ipad. #DundeePublicH
- 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. #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. #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
- 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. Money Do you know how much an RCT costs to run? http://www.moneysigns.net/british-money-bills/ #DundeePublicH
- 12. We already do things http://en.wikipedia.org/wiki/File:Illustration _Salix_alba0.jpg #DundeePublicH
- 15. #DundeePublicH
- 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
- 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. The structure of a cohort study http://sph.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/#DundeePublicH
- 20. The structure of a cohort study http://sph.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/#DundeePublicH
- 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. Types of cohort study Population Exposed Outcome No outcome Not exposed Outcome No outcome Prospective Retrospective #DundeePublicH
- 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. #DundeePublicH
- 25. When is a general population not a general population? #DundeePublicH
- 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
- 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. #DundeePublicH
- 30. #DundeePublicH
- 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. Risk Disease status Present Absent Present a b Absent c d #DundeePublicH
- 33. Relative risk (RR) Pevent when exposed RR= Pevent when not exposed a/a+b = d/c+d #DundeePublicH
- 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. Interpreting a Relative Risk Risk Ratio = 1#DundeePublicH
- 36. Interpreting a Relative Risk Risk Ratio <1 Logarithmic scale Incidence in exposed < incidence in control #DundeePublicH
- 37. Interpreting a Relative Risk Risk Ratio >1 Logarithmic scale Incidence in exposed > incidence in control #DundeePublicH
- 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. Which is sexier? 50% reduction in risk Reduction in risk from 1 in 200,000 to 1 in 400,000 #DundeePublicH
- 40. Which is sexier? 50% reduction in risk Reduction in risk from 1 in 200,000 to 1 in 400,000 #DundeePublicH
- 46. P.S. Calculating Absolute Risk: Risk in exposed group – Risk in non exposed group #DundeePublicH
- 47. #DundeePublicH
- 48. Do mobile phones cause brain cancer? http://e-healthdiary.com/category/cancer/brain-cancer#DundeePublicH
- 49. How do you answer the question? Randomised controlled trial Cohort? #DundeePublicH
- 50. How a case control study works http://library.downstate.edu/EBM2/2500.htm#DundeePublicH
- 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. 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
- 54. #DundeePublicH
- 56. #DundeePublicH
- 57. Do mobile phones cause brain cancer? http://e-healthdiary.com/category/cancer/brain-cancer#DundeePublicH
- 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. #DundeePublicH
- 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. Risk Disease status Present Absent Present a b Absent c d #DundeePublicH
- 62. Odds Ratio (OR) Oddsevent when exposed OR= Oddsevent when not exposed a/c = b/d axd = bxc #DundeePublicH
- 63. Interpreting an Odds Ratio Odds Ratio = 1#DundeePublicH
- 64. Interpreting an Odds Ratio Odds Ratio <1 Logarithmic scale Risk in exposed < risk in control #DundeePublicH
- 65. Interpreting an Odds Ratio Odds Ratio >1 Logarithmic scale Risk in exposed > risk in control #DundeePublicH
- 66. Odds ratios are not odds! #DundeePublicH
- 67. Do mobile phones cause brain cancer? http://e-healthdiary.com/category/cancer/brain-cancer#DundeePublicH
- 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. Results Odds ratio for regular users compared with not-regular-users = 0.79 20% LESS likely #DundeePublicH
- 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. 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/
- 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. 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. 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. #DundeePublicH Research methods: the Evidence Pyramid http://www.mstrust.org .uk/professionals/infor mation/wayahead/articl es/16032012_08.jsp