Healthy Worker Effect (HWE)

An annoying methodological aspect in occupational studies?

Khaldoun NijemNovember 25, 2002

Contents

Definition of HWE Historical background Importance of HWE Core of HWE Factors affecting HWE Avoiding HWE Summary and conclusion

Definition

“A phenomenon observed initially in studies of occupational diseases: workers usually exhibit lower overall death rates than the general population, because the severely ill and chronically disabled are ordinarily excluded from employment”

Last, 1995

Definition

“HWE refers to the consistent tendency for the actively employed people to have a more favorable mortality experience than the population at large”

(McMichael 1987)

Example

Mortality (SMR, all causes) of gas workers compared with national experience

heavy exposure intermediate no exposure105 90 84

(Doll et al 1965)

What is standardization? A set of techniques used to remove

as far as possible the effects of difference in age or other confounding variables when comparing two or more populations.

Standardization methods:1- Direct standardization2- Indirect standardization (SMR)

Last, 1995

Standardized mortality ratio (SMR)

The ratio of the number of deaths observed in the study group or population to the number that would be expected if the study population had the same specific rates as the standard population, multiplied by 100

Last, 1995

Example

Mortality (SMR, all causes) of gas workers compared with national experience

heavy exposure intermediate no exposure105 90 84

(Doll et al 1965)

Historical Background The phenomenon was first

observed by Ogle (1885) who found that “the more vigorous occupations had relatively lower mortality rate as compared with the death-rates in occupations of an easier character or the unemployed”

(Arrighi & Hertz-Picciotto 1994)

The term “healthy worker effect” was first used by McMichael et al in 1974

Historical Background, contd

Goldsmith (1975) pointed out that most industrially-employed cohorts should be expected to have better life expectancy than unemployed persons

Historical Background, contd

Fox and Collier examined the healthy worker survivor effect in terms of the standardized mortality ratio (SMR) with the general population as reference

(Fox & Collier 1976)

Historical Background, contd

SMRs close to unity (100) was used as an indication of absence or a low degree of the HWE

(Fox & Collier 1976, Doll 1988)

Historical Background, contd

Importance of HWE

A common problem?

Computer searchMedline: 59 articles with HWE in

titleOccupational databases:

additional 20 articles with HWE in title

General Medline search: more than 400 hits

Reduce SMR by 20-30%

Example, SMR, all causes of gasworkers compared with national experience

heavy exposure intermediate no exposure105 90 84

(Doll et al 1965)

A serious problem?

Reduce SMR by 20-30%

Example, SMR, all causes of gas workers compared with national experience

heavy exposure intermediate no exposure

105 90 84

(Doll et al 1965)

A serious problem?

It may partially or completely mask excess mortality or morbidity that is caused by harmful exposure

A serious problem

Core of the HWE

Although the HWE is well recognized, it has been considered to be a poorly defined phenomenon

(Li & Sung 1999)

Core of the HWE

Core of the HWE

Selection bias? Confounding? Mixture of different

biases? Comparison problem?

What is Selection Bias?

“ Error due to systematic differences in characteristics between those who are selected for study and those who are not.”

Last 1995

Is HWE a Selection Bias?

“HWE is the most common selection bias in occupational studies and occurs because relatively healthy individuals are likely to gain employment and to remain employed”. (McMicheal 1987; Checkoway et al 1989)

HWE refers to the initial hiring into the workforce and the subsequent factors which influence continuing employment

(Arrighi & Hertz-Picciotto 1994)

Confounding

“ A situation in which a measure of the effect of an exposure on risk is distorted because of the association of exposure with other factor(s) that influence the outcome under study.”

Last 1995

Example of Confounding

Coffee drinking Heart disease

Cigarette Smoking

Is HWE a case of confounding?

Monson (1986) considered HWE to be an example of confounding: “The confounding factor is the (unmeasured) health status of the group of employees”

HWE as a case of confounding

Employment in the industry death

Good health status