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SEARO – CSR Training on Outbreak Investigation Two-by-Two Table Cornerstone in epidemiological research

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Learning Objectives At the end of the presentation, the participants will be able to: –Use two-by-two table for computation of epidemiological data –Define measures of associate and indicate use

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation ‘Every epidemiologic study can be summarized in a two-by-two table.’ - H. Ory

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Two-by-two Table Cornerstone of all epidemiological research Often the first thing one draws up when starting to investigate some data

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Two-by-two Table Exposed Not exposed ab c d not illill Total b + da + c c + d a + b

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation The legend a = ill and exposed b = well and exposed d = well and not exposed c = ill and not exposed exposed not exposed not illill ab dc

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Total number exposed ab cd exposed not exposed not illill a + b c + d b + da + c Total number not exposed Total number of cases/ill Total number of controls/not ill The legend Total

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Example: New Year’s Dinner Outbreak Food itemIllNot Ill EatenNot EatenEatenNot Eaten Cheesecake4119 Quiche2382 Swiss roll3246 Chocolate cake 1428 Cheese dip4173

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Food itemIllNot Ill EatenNot EatenEatenNot Eaten Cheesecake4119 Eat cheesecake Did not eat cheesecake 41 19 not illill 15 b + da + c c + d a + b4 + 1 = 5 1 + 9 = 10 4 + 1 = 51 + 9 = 10 Total

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Attack Rate (AR) = Number of population at risk Number of people who are ill Attack rate close to 1 might give us an idea of possible item in question.

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Eat cheesecake Did not eat cheesecake 41 19 not illill 15 4 + 1 = 5 1 + 9 = 10 4 + 1 = 51 + 9 = 10 AR = 5 15 = 0.33 Total = AR = 4 5 = 0.8 exposed AR = 1 10 = 0.1 not exposed

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Using Denominator Infection by S.Typhimurium Attack Rate by age group, Jura, May-June 1997 Age group (years) Number of Cases <12 1 – 536 6 – 1422 15 – 6429 > 659 Total98

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Using Denominator Infection by S.Typhimurium, Attack Rate by age group, Jura, may-June 1997 Age Group (years) Number of CasesPopulationAttack Rate/ 100,000 <12522383 1 – 53616,014225 6 – 142230,38572 15 – 6429157,98918 > 65941,94822 Total98246,85840

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Use of Two-By-Two Table in Measures of Association

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Relative Risk = the risk in the exposed group divided by the risk in the unexposed group = 8 Relative Risk (RR) = c/(c +d) a/(a + b) = 0.1 0.8 Cohort studies Exposed Not exposed 41 19 not illill b + d a + c c + d a + b 4 + 1 = 5 1 + 9 = 10 4 + 1 = 51 + 9 = 10 Total = 15 RR = 1 no association RR > 1 there is association RR < 1 some way protective

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation Odds Ratio = the odds for cases divided by odds for control = 2.25 Case control studies Exposed Not exposed 3025 3260 controlcase b + d a + c c + d a + b 30 + 25 = 55 32 + 60 = 92 30+32 = 62 25+60 = 85 When true size of the exposed and unexposed groups is unknown e.g., cholera cases and control and exposure to shallow well N’Djamena, Chad, July 2001 Odds Ratio (OR) = bc ad = 25 x 32 30 x 60 = 800 1800 OR = 1 no association OR > 1 there is association Open well

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation References Giesecke, J. Modern Infectious Disease Epidemiology, 2 nd ed. Oxford University Press. 2002. Gregg, GB. Field Epidemiology, 2 nd ed. Oxford University Press. 2002. Hennekens, C.H., Buring, J.E., Epidemiology in Medicine. 1 st ed. 3 rd printing. Little Brown and Company, 1987.

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Two-by-Two Table SEARO – CSR Training on Outbreak Investigation

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