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Two-by-Two Table

Two-by-Two Table. Cornerstone in epidemiological research. 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

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  1. Two-by-Two Table Cornerstone in epidemiological research

  2. 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

  3. ‘Every epidemiologic study can be summarized in a two-by-two table.’ - H. Ory

  4. Two-by-two Table • Cornerstone of all epidemiological research • Often the first thing one draws up when starting to investigate some data

  5. Two-by-two Table ill not ill a b Exposed a + b c d Not exposed c + d Total a + c b + d

  6. The legend ill not ill exposed not exposed a = ill and exposed a b b = well and exposed c = ill and not exposed c d d = well and not exposed

  7. ill not ill a b exposed c d not exposed The legend Total number exposed a + b c + d Total number not exposed Total a + c b + d Total number of controls/not ill Total number of cases/ill

  8. Example: New Year’s Dinner Outbreak

  9. ill not ill 4 1 Eat cheesecake 4 + 1 = 5 a + b 1 9 Did not eat cheesecake c + d 1 + 9 = 10 Total 15 4 + 1 = 5 a + c b + d 1 + 9 = 10

  10. Number of people who are ill Attack Rate (AR) = Number of population at risk Attack rate close to 1 might give us an idea of possible item in question.

  11. 1 4 AR = AR = 5 exposed not exposed AR = 10 5 15 = 0.1 = 0.8 = 0.33 ill not ill 4 1 Eat cheesecake 4 + 1 = 5 1 9 Did not eat cheesecake 1 + 9 = 10 Total = 15 4 + 1 = 5 1 + 9 = 10

  12. Using DenominatorInfection by S.TyphimuriumAttack Rate by age group, Jura, May-June 1997

  13. Using Denominator Infection by S.Typhimurium, Attack Rate by age group, Jura, may-June 1997

  14. Use of Two-By-Two Table inMeasures of Association

  15. ill not ill a + b 4 1 a/(a + b) Relative Risk (RR) = Exposed 4 + 1 = 5 c/(c +d) 1 9 c + d Not exposed 0.8 1 + 9 = 10 = 0.1 a + c b + d 4 + 1 = 5 1 + 9 = 10 Cohort studies Total = 15 RR = 1 no association RR > 1 there is association RR < 1 some way protective = 8 Relative Risk = the risk in the exposed group divided by the risk in the unexposed group

  16. case control a + b 30 25 Exposed 30 + 25 = 55 32 60 c + d Not exposed 32 + 60 = 92 a + c b + d 25+60 = 85 30+32 = 62 ad Odds Ratio (OR) = bc 1800 30 x 60 = = 25 x 32 800 Case control studies 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 Open well Open well OR = 1 no association OR > 1 there is association = 2.25 Odds Ratio = the odds for cases divided by odds for control

  17. References Giesecke, J. Modern Infectious Disease Epidemiology, 2nd ed. Oxford University Press. 2002. Gregg, GB. Field Epidemiology, 2nd ed. Oxford University Press. 2002. Hennekens, C.H., Buring, J.E., Epidemiology in Medicine. 1st ed. 3rd printing. Little Brown and Company, 1987.

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