Epidemiology Lecture 7

8/8/2019 Epidemiology Lecture 7

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Measures of Public

Health Impact


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Learning Objectives:

1. Calculate and interpret measures of public

health impact:

--- Attributable risk

--- Attributable risk percent

--- Population attributable risk

--- Population attributable risk percent

2. Differentiate between attributable risk and

relative risk.

3. Differentiate between high-risk and

population-based approaches to disease

prevention.


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Measures of Public Health ImpactMeasures of Public Health Impact

Attributable Risk (AR) Number

Attributable Risk Percent (AR%) Percentage

Population Attributable Risk (PAR) Number

Population Attributable Risk Percent

(PAR%) Percentage


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IMPORTANT!

They all assume (require) that a cause-

effect relationship exists between the

exposure and the outcome.


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Relative Risk vs. Attributable RiskRelative Risk vs. Attributable Risk

Relative Risk: Measure of the strength of

association, and indicator used to

assess the possibility of a causal

relationship.

Attributable Risk: Measure of the

potential forprevention of disease if theexposure could be eliminated (assuming

a causal relationship).


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Relative Risk vs. Attributable RiskRelative Risk vs. Attributable Risk

Relative Risk:

Etiology

Attributable Risk:

Policy decisions

Funding decisions(e.g. prevention programs)


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Attributable Risk:

Refers to EXPOSED persons.

Population Attributable Risk:

Refers to both EXPOSED and

NONEXPOSED persons.



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Attributable Risk (AR)Attributable Risk (AR)

Among the EXPOSED:

How much of the disease that occurs can

be attributed to a certain exposure?AR

AR%

This is of primary interest to the practicing

clinician.


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AR = Iexposed Inonexposed = Risk Difference

Smoke Yes No

Yes 84 2916 3000

No 87 4913 5000

Develop CHD ISM = 84 / 3000

= 0.028 = 28.0 / 1000

INS = 87 / 5000

= 0.0174 = 17.4 / 1000

(background risk)

AR = (28.0 17.4) / 1000 = 10.6 / 1000


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AR = (28.0 17.4) / 1000 = 10.6 / 1000

Among SMOKERS, 10.6 of the 28/1000

incident cases of CHD are attributedto the fact that these people smoke

Among SMOKERS, 10.6 of the 28/1000

incident cases of CHD that occurcould be prevented if smoking were

eliminated.


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Attributable Risk Percent (AR%)Attributable Risk Percent (AR%)

AR% = (Iexposed Inonexposed) / Iexposed= Etiologic fraction

Smoke Yes No

Yes 84 2916 3000

No 87 4913 5000

Develop CHD

AR% = (28.0 17.4) / 28.0 = 37.9%

ISM = 84 / 3000

= 0.028 = 28.0 / 1000

INS = 87 / 5000

= 0.0174 = 17.4 / 1000

(background risk)


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Attributable Risk Percent (AR%)Attributable Risk Percent (AR%)

AR% = (28.0 17.4) / 28.0 = 37.9%

Among SMOKERS, 38% of the morbidity

from CHD may be attributed tosmoking

Among SMOKERS, 38% of the morbidity

from CHD could be prevented ifsmoking were eliminated.


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Discussion QuestionDiscussion Question

If 38% of the morbidity from CHD is due

to smoking, it seems as if we found many

factors causally related to CHD,

the attributable risk for all factors

combined could exceed 100%

How can this be?


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Sufficient

Cause ISufficient

Cause II

Sufficient

Cause III

U

A B

U

A E

U

B E

Accounts for

50% of dx cases

Accounts for

30% of dx cases

Accounts for

20% of dx cases

If we can prevent any of the factors:

U = 100% reduction in disease occurrence

A = 80% reduction in disease occurrence

B = 70% reduction in disease occurrence

E = 50% reduction in disease occurrence


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If we can prevent any of the factors:

U = 100% reduction in disease occurrence

A = 80% reduction in disease occurrence

B = 70% reduction in disease occurrenceE = 50% reduction in disease occurrence

(U + A + B + E) =300%

Discussion QuestionDiscussion Question

Hence, because of multi-factorial etiology andmultiple sufficient causes (mechanisms), the

sum of the individual ARs for each causal

factor can exceed 100%.


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Population Attributable Risk (PAR)Population Attributable Risk (PAR)

Among the EXPOSED and NONEXPOSED(e.g. total population):

How much of the disease that occurs

can be attributed to a certain exposure?

PAR

PAR%

This of interest to policy makers and thoseresponsible for funding preventionprograms.


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PAR and PAR%PAR and PAR%

Example:

We want to estimate how much of

the burden of diabetes amongKarachites is attributed to obesity.


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PAR and PAR%PAR and PAR%

CAUTION!

In order to calculate PAR and PAR%, we

have to be reasonably sure that theresults of the study can be generalized

to the population of Karachi

(e.g the incidence rates drawn from thesample approximate the incidence rates

in the entire population).


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PAR = Itotal Inonexposed

WeightYes No

Obese 850 3650 4500

Slim 250 5250 5500

Diabetes IT = 1100 / 10000

= 0.11 = 110 / 1000

INE = 250 / 5500

= 0.0455 = 45.5 / 1000

(background risk)

PAR = (110 45.5) / 1000 = 64.5 / 1000

1100 8900 10000


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PAR = (110 45.5) / 1000 = 64.5 / 1000

In Karachi, 64.5 of the 110/1000 incident

cases of diabetes are attributed toobesity

In Karachi, 64.5 of the 110/1000 incident

cases of diabetes that occur could be

prevented with sufficient weight loss.


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Population Attributable Risk PercentPopulation Attributable Risk Percent

PAR% = (Itotal Inonexposed) / Itotal

Weight Yes No

Obese 850 3650 4500

Slim 250 5250 5500

Diabetes

PAR% = (110 45.5) / 110 = 58.6%

1100 8900 10000

IT = 1100 / 10000

= 0.11 = 110 / 1000

INE = 250 / 5500

= 0.0455 = 45.5 / 1000

(background risk)


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Population Attributable Risk PercentPopulation Attributable Risk Percent

PAR% = (110 45.5) / 110 = 58.6%

In Karachi, 59% of the cases of diabetes

may be attributed to obesity in thepopulation

In Karachi, 59% of the cases of diabetes

could be prevented if Karachiresidents lost sufficient weight.


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NOTE!Both attributable and population

attributable risks should be cautiously

interpreted.

In reality, even if an exposure is causal,

we do not know whether it truly

contributed to disease occurrence inall exposed persons -- in some

exposed persons, other causal factors

may have been entirely responsible.


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Calculating Measures ofCalculating Measures of

Public Health ImpactPublic Health Impact(Case(Case--Control Studies)Control Studies)


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They are based on measures of incidence.

We can calculate incidence measures from

case-control studies only under special

circumstances.

Therefore, the AR and PAR cannot usually

be calculated from case-control data.

However, for most case-control studies, we

can calculate the AR% and PAR%.



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AR% (CaseAR% (Case--Control Studies)Control Studies)

(OR 1)

AR% = ----------- x 100

OR


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Example: AR% (CaseExample: AR% (Case--Control Studies)Control Studies)

Smoke Yes No

Yes 160 120

No 90 200

Case-control study to evaluate the impact ofsmoking as related to bladder cancer.

Bladder Cancer(160 / 90)

OR = ------------

(120 / 200)

= 2.96


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Question: Among smokers, what proportion(percent) of bladder cancer cases can be

attributed to their smoking habit?

(OR 1)

AR% = ----------- x 100

OR

AR% = ((2.96 1) / 2.96) x 100 = 66.2%


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66% of bladder cancer cases amongsmokers can be attributed to theirsmoking.

66% of bladder cancer cases amongsmokers could be prevented if theyhad never taken up smoking.

(Assuming there is a causalassociation between smoking and thedevelopment of bladder cancer).


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Issues in Prevention PolicyIssues in Prevention Policy

An important question in prevention iswhether the approach should target

specific groups known to be at high

risk, or extend to the general

population as a whole.

This depends largely on the nature of

the exposure/disease relationship,and the distribution of the exposure in

the population.

Epidemiology Lecture 7

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