chap8-eks-2ed.ppt bhhhnHuhhjjjjjjjjjjjjjj

Ghhhjuiiiiuuuhhhv bbghhhhhhhhhhhhhhhhhhhhhhhhhhhhhh


















Mm
Mm
Mm

1. Epidemiology Kept Simple
Chapter 8
Measures of Association &
Potential Impact

2. Chapter 8 2Gerstman
Important Jargon
•Exposure (E)  an explanatory factor; any
potential health determinant; the independent
variable
•Disease (D)  the response; any health-related
outcome; the dependent variable
•Measure of association (syn. measure of
effect)  a statistic that quantifies the
relationship between an exposure and a disease
•Measure of potential impact  a statistic
that quantifies the potential impact of removing a
hazardous exposure

3. Chapter 8 3Gerstman
Arithmetic (αριθμός)
Comparisons
•Measures of association are
mathematical comparisons
•Mathematic comparisons can
be done in absolute terms or
relative terms
•Let us start with this ridiculously
simple example:
•I have $2
•You have $1
"For the things of this world
cannot be made known
without a knowledge of
mathematics."- Roger Bacon

4. Chapter 8 4Gerstman
Absolute Comparison
•In absolute terms, I
have $2 – $1
= $1 more than you
•Note: the absolute
comparison was made
with subtraction
It is as simple as that…

5. Chapter 8 5Gerstman
Relative Comparison
•Recall that I have $2 and
you have $1.
•In relative terms,
I have $2 ÷ $1 = 2, or
“twice as much as you”
•Note: relative comparison
was made by division

6. Chapter 8 6Gerstman
•Suppose, I am exposed to a risk
factor and have a 2% risk of disease.
•You are not exposed and you have a
1% risk of the disease.
Applied to Risks
•Of course we are assuming we are
the same in every way except for this
risk factor.
•In absolute terms, I have 2% – 1%
= 1% greater risk of the disease
•This is the risk difference

7. Chapter 8 7Gerstman
•In relative terms I have
2% ÷ 1% = 2,
or twice the risk
•This is the relative risk
associated with the
exposure
Applied to Risks

8. Chapter 8 8Gerstman
Terminology
For simplicity sake, the terms
“risk” and “rate” will be applied
to all incidence and prevalence
measures.

9. Chapter 8 9Gerstman
Risk Difference
Risk Difference (RD)  absolute effect
associated with exposure
01
RRRD 
where
R
1 ≡ risk in the exposed group
R
0 ≡ risk in the non-exposed group
Interpretation: Excess risk in absolute terms

10. Chapter 8 10Gerstman
Relative Risk
Relative Risk (RR)  relative effect associated
with exposure or the “risk ratio”
0
1
R
R
RR
where
R
1 ≡ risk in the exposed group
R
0 ≡ risk in the non-exposed group
Interpretation: excess risk in relative terms.

11. Chapter 8 11Gerstman
Example
Fitness & Mortality (Blair et al., 1995)
•Is improved fitness associated
with decreased mortality?
•Exposure ≡ improved fitness
(1 = yes, 0 = no)
•Disease ≡ death
(1 = yes, 0 = no)
•Mortality rate, group 1:
R
1
= 67.7 per 100,000 p-yrs
•Mortality rate, group 0:
R
0
= 122.0 per 100,000 p-yrs

12. Chapter 8 12Gerstman
Example
Risk Difference
01
RRRD 
The effect of the exposure (improved fitness) is to decrease
mortality by 54.4 per 100,000 person-years
What is the effect of improved fitness on mortality in
absolute terms?
yrs-p 100,000
0.122

yrs-p 100,000
7.67

yrs-p 100,000
4.54


13. Chapter 8 13Gerstman
Example
Relative Risk
0
1
R
R
RR
What is the effect of improved fitness on mortality in
relative terms?
55.0
yrs-p 100,000per 0.122
yrs-p 100,000per 7.67

The effect of the exposure is to cut the risk almost in half.

14. Chapter 8 14Gerstman
Designation of Exposure
•Switching the designmation of
“exposure” does not materially
affect interpretations
•For example, if we had let
“exposure” ≡ failure to improve
fitness
•RR = R
1
/ R
0

= 122.0 / 67.7
= 1.80
(1.8 times the risk in the
exposed group (“almost
double”)

15. Chapter 8 15Gerstman
2-by-2 Table Format
Disease +Disease −Total
Exposure + A
1
B
1
N
1
Exposure – A
0 B
0 N
0
TotalM
1
M
0 N
For person-time data: let N
1
≡ person-time in group 1 and N
0
≡
person-time in group 0, and ignore cells B
1
and B
0
1
1
1
N
A
R
0
0
0
N
A
R

16. Chapter 8 16Gerstman
Fitness Data, table format
Fitness
Improved?
Died Person-years
Yes 25 -- 4054
No 32 -- 2937
67.61000,10
4054
25
1
1
1 
N
A
R
95.108000,10
2937
32
0
0
0 
N
A
R
Rates per 10,000 person-years

17. Chapter 8 17Gerstman
Food borne Outbreak Example
Disease +Disease −Total
Exposure + 63 25 88
Exposure –

1 6 7
Total 64 31 95
7159.0
88
63
1
1
1

N
A
R 1429.0
7
1
0
0
0 
N
A
R
Exposure ≡ eating a particular dish
Disease ≡ gastroenteritis

18. Chapter 8 18Gerstman
Food borne Outbreak Data
71
8863
0
1

R
R
RR
1429.0
7159.0
 01.5
Exposed group had 5 times the risk
Disease +Disease −Total
Exposure + 63 25 88
Exposure – 1 6 7
Total 64 31 95

19. Chapter 8 20Gerstman
What do you do when you have
multiple levels of exposure?
Compare rates to least exposed “reference” group
LungCA Rate
(per 100,000 person-years)
RR
Non-smoker (0) 10 1.0 (ref.)
Light smoker (1) 52 5.2
Mod. smoker (2) 106 10.6
Heavy sm. (3) 224 22.4
2.5
01
25
0
1
1 
R
R
RR 6.10
01
106
0
2
2

R
R
RR

20. Chapter 8 21Gerstman
The Odds Ratio
•When the disease is
rare, interpret the
same way you
interpret a RR
•e.g. an OR of 1
means the risks are
the same in the
exposed and
nonexposed groups
D+D−Total
E+A
1B
1 N
1
E−A
0
B
0
N
0
TotalM
1M
0N
01
01
00
11
AB
BA
BA
BA
OR 
“Cross-product ratio”
Similar to a RR, but based on odds rather than risks

21. Chapter 8 22Gerstman
Odds Ratio, Example
Milunsky et al, 1989, Table 4
NTD = Neural Tube Defect
NTD+ NTD−
Folic Acid+10 10,703
Folic Acid−39 11,905
01
01
AB
BA
OR
Exposed group had 0.29 times (about a quarter)
the risk of the nonexposed group
39703,10
905,1110


 29.0

22. Chapter 8 23Gerstman
Measures of Potential Impact
•These measures
predicted impact of
removing a hazardous
exposure from the
population
•Two types
–Attributable fraction in
exposed cases
–Attributable fraction in
the population as a
whole

23. Chapter 8 24Gerstman
Attributable Fraction
Exposed Cases (AF
e
)
RR
RR
AF
e
1
:formula Equivalent


1
01
:formula alDefinition
R
RR
AF
e


Proportion of exposed cases averted with
elimination of the exposure

24. Chapter 8 25Gerstman
Example: AF
e
RR of lung CA associated with moderate smoking
is approx. 10.4. Therefore:
RR
RR
AF
e
1

Interpretation: 90.4% of lung cancer in moderate
smokers would be averted if they had not smoked.
904.
4.10
14.10




25. Chapter 8 26Gerstman
Attributable Fraction,
Population (AF
p)
population nonexposedin rate
rate overall
where
:formula alDefinition
0
0




R
R
R
RR
AF
p
Proportion of all cases averted with
elimination of exposure from the population

26. Chapter 8 27Gerstman
AF
p equivalent formulas
populationin exposure of prevalence where

)1(1
)1(




e
e
e
p
p
RRp
RRp
AF
exposed are that cases of proportion where



c
cep
p
pAFAF

27. Chapter 8 28Gerstman
AF
p
for Cancer Mortality,
Selected Exposures
Exposure Doll & Peto, 1981 Miller, 1992
Tobacco 30% 29%
Dietary 35% 20%
Occupational 4% 9%
Repro/Sexual 7% 7%
Sun/Radiation 3% 1%
Alcohol 3% 6%
Pollution 2% -
Medication 1% 2%
Infection 10% -