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

SEARO – CSR Training on Outbreak Investigation

Two-by-Two TableCornerstone in epidemiological

research

Two-by-Two TableSEARO – 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


‘Every epidemiologic study can be summarized in a two-

by-two table.’ - H. Ory


Two-by-two Table

• Cornerstone of all epidemiological research

• Often the first thing one draws up when starting to investigate some data


Two-by-two Table

Exposed

Not exposed

a b

c d

not illill

Totalb + d

a + c

c + d

a + b


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

a b

dc


Total number exposeda b

c d

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


Example: New Year’s Dinner Outbreak

Food item Ill Not Ill

Eaten Not Eaten Eaten Not Eaten

Cheesecake 4 1 1 9

Quiche 2 3 8 2

Swiss roll 3 2 4 6

Chocolate cake

1 4 2 8

Cheese dip 4 1 7 3


Food item Ill Not Ill

Eaten Not Eaten Eaten Not Eaten

Cheesecake 4 1 1 9

Eat cheesecake

Did not eat cheesecake

4 1

1 9

not illill

15b + d

a + c

c + d

a + b4 + 1 = 5

1 + 9 = 10

4 + 1 = 5 1 + 9 = 10

Total


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.


Eat cheesecake

Did not eat cheesecake

4 1

1 9

not illill

15

4 + 1 = 5

1 + 9 = 10

4 + 1 = 5

1 + 9 = 10

AR =5

15

= 0.33

Total =

AR =4

5

= 0.8

exposed AR =1

10= 0.1

not exposed


Using DenominatorInfection by S.Typhimurium

Attack Rate by age group, Jura, May-June 1997

Age group (years)

Number of Cases

<1 2

1 – 5 36

6 – 14 22

15 – 64 29

> 65 9

Total 98


Using Denominator Infection by S.Typhimurium,

Attack Rate by age group, Jura, may-June 1997

Age Group (years)

Number of Cases

Population Attack Rate/ 100,000

<1 2 522 383

1 – 5 36 16,014 225

6 – 14 22 30,385 72

15 – 64 29 157,989 18

> 65 9 41,948 22

Total 98 246,858 40


Use of Two-By-Two Table inMeasures of Association


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 studiesExposed

Not exposed

4 1

1 9

not illill

b + da + c

c + d

a + b4 + 1 = 5

1 + 9 = 10

4 + 1 = 5

1 + 9 = 10

Total = 15

RR = 1 no association

RR > 1 there is association

RR < 1 some way protective


Odds Ratio = the odds for cases divided by odds for control

= 2.25

Case control studiesExposed

Not exposed

30 25

32 60

controlcase

b + da + 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

1800OR = 1 no association

OR > 1 there is association

Open well

Open well


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.


Documents

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