Healthy Worker Effect (HWE)

An annoying methodological aspect in occupational studies?

Khaldoun NijemNovember 25, 2002

Today I will talk about an occupational epidemiological phenomena known as HWE. This phenomena was under debate and a lot of discussion during the last years. Some epidemiologist regard HWE as an ordinary method problems, while others considered it as a field of science by itself.

ContentsDefinition of HWEHistorical backgroundImportance of HWECore of HWEFactors affecting HWEAvoiding HWESummary and conclusion

In this lecture I will talk about different diffeniton of HWE and the historical back ground. What its important to understand this phenomena and what is the core of HWE, by that I mean the crux of the problem. I will also talk about the different factors affecting the phenomena and how to minimize it then at the end of this lecture I will make the summay and main conclusion.

DefinitionA 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

DefinitionHWE refers to the consistent tendency for the actively employed people to have a more favorable mortality experience than the population at large (McMichael 1987)

Another difinition by Mcmichaelt who first give its name is (read the slide). However, other occupational epidemiologists describe HWE as the reduction of mortality or morbidity of occupational cohorts when compared with the general population. (Next slide is an example)

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

heavy exposure intermediate no exposure105 90 84

(Doll et al 1965)

As an example is the study of Doll and coworkers on gas workers exposed to carbonised coal. They measured SMR for groups of gasworkers with different exposure. .

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

SMR is an example on indirect standardization where it used to eliminate possible effect of age differences by comparing a cohort of workers with the general population

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 100Last, 1995

The standardized mortality ratio is defined as the ratio between observed mortality rate among workers exposed to certain hazards and expected mortality rate of the same workers standarized with the general population.

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

heavy exposure intermediate no exposure105 90 84

Returning to the previous example. The unexposed workers had SMR less than 100

(Doll et al 1965)

Historical BackgroundThe 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 phenomena was observed in 1885 when William Ogle found that mortality rates is depending on the difficulty of the work where some occupations may repeal, while others may attract the workers

The term healthy worker effect was first used by McMichael et al in 1974 Almost One hundred years later Mcmichael coined the term HWE to describe this phenomena.

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)Trying to measure the magnitude of HWE, fox and collier used Standardized Mortality ratio .

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)They considered MRS close to unity as indication of low degree of HWE. Historical Background, contd

Importance of HWE

A common problem?

Computer searchMedline: 59 articles with HWE in titleOccupational databases: additional 20 articles with HWE in titleGeneral Medline search: more than 400 hits

The first question is HWE is common. Any occupational study looking for the worker health could potentially faced this problem. When we made computer search in medline from the year 74; when it was first named-we found 59 articles with HWE in its title. an extra search in other occupational database result in additional 20 articles with HWE in the title. More than 400 articles contain HWE in the content. So its a common problem

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?

Is it a serious bias?. Actually, Most of the study indicated that HWE will reduce the association between exposure and outcome by an average of 20-30 percent. I will use the same example as I showed earlier, the British gasworkers:.(see the slide and point to no exposure and move to next slidde next slide is an example).

Reduce SMR by 20-30%

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

heavy exposure intermediate no exposure105 90 84

(Doll et al 1965)A serious problem?

It may partially or completely mask excess mortality or morbidity that is caused by harmful exposureA serious problem

So this reduction in SMR could be described as a negative effect of exposure-outcome association or in other terms it might said that the condition among the workers is good and no harmful effect was seen. And of course it is not true..

Core of the HWE

Trying to under stand HWE was not easy since there was a lot of explaination and debate about its nature, a good example could be seen in Choi 1992 when he described the differences in opion regarding HWE. The article included the openion of nine different participents familiar with 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 HWESelection bias?Confounding?Mixture of different biases?Comparison problem?

Some of them considered HWE as a source of selection bias, others considered it as a confounding . A third group considered it as a mixture of both. While others considered it as a coparison problem

Some of them considered HWE as a source of selection bias, others considered it as a confounding . A third group considered it as a mixture of both. While others considered it as a coparison 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)

According to this difinition is HWE is a source of selection bias. McMichael considered it a source of selection bias because the characteristic of the occupational cohort selected to participate in a work are differ they are healthy-from the general population which contains healthy and unhealthy casescause only healthy workers will gain and remain employed.

HWE refers to the initial hiring into the workforce and the subsequent factors which influence continuing employment (Arrighi & Hertz-Picciotto 1994)

The selection bias occurred from the initial choosing of workers (mainly healthy workers) and the factors that influence the contineouty of work such as leaving thework because of sickness.

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

An example on confounding is the following diagram. The confounder should associate with both exposure and outcome

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

According to Monson HWE is considered as a case of confounding. The confounder is the unmeasured health status. also He considered the appearance of selection bias depends on the action of the investigators, whereas confounding is a characteristic of the study population.

HWE as a case of confounding

Employment in the industry death

Good health status

Going back to our example GHS is associated with exposure (Employment in industry and associated with outcome death

*Today I will talk about an occupational epidemiological phenomena known as HWE. This phenomena was under debate and a lot of discussion during the last years. Some epidemiologist regard HWE as an ordinary method problems, while others considered it as a field of science by itself. *In this lecture I will talk about different diffeniton of HWE and the historical back ground. What its important to understand this phenomena and what is the core of HWE, by that I mean the crux of the problem. I will also talk about the different factors affecting the phenomena and how to minimize it then at the end of this lecture I will make the summay and main conclusion.*How we define HWE. Last define the phenomena as*Another difinition by Mcmichaelt who first give its name is (read the slide). However, other occupational epidemiologists describe HWE as the reduction of mortality or morbidity of occupational cohorts when compared with the general population. (Next slide is an example)*As an example is the study of Doll and coworkers on gas workers exposed to carbonised coal. They measured SMR for groups of gasworkers with different exposure. . *SMR is an example on indirect standardization where it used to eliminate possible effect of age differences by comparing a cohort of workers with the general population*The standardized mortality ratio is defined as the ratio between observed mortality rate among workers exposed to certain hazards and expected mortality rate of the same workers standarized with the general population.*Returning to the previous example. The unexposed workers had SMR less than 100*The phenomena was observed in 1885 when William Ogle found that mortality rates is depending on the difficulty of the work where some occupations may repeal, while others may attract the workers*Almost One hundred years later Mcmichael coined the term HWE to describe this phenomena.*Read the slide)*Trying to measure the magnitude of HWE, fox and collier used Standardized Mortality ratio .*They considered MRS close to unity as indication of low degree of HWE. *To see if this phenomena is important different questions could be asked*The first question is HWE is common. Any occupational study looking for the worker health could potentially faced this problem. When we made computer search in medline from the year 74; when it was first named-we found 59 articles with HWE in its title. an extra search in other occupational database result in additional 20 articles with HWE in the title. More than 400 articles contain HWE in the content. So its a common problem*Is it a serious bias?. Actually, Most of the study indicated that HWE will reduce the association between exposure and outcome by an average of 20-30 percent. I will use the same example as I showed earlier, the British gasworkers:.(see the slide and point to no exposure and move to next slidde next slide is an example). * *So this reduction in SMR could be described as a negative effect of exposure-outcome association or in other terms it might said that the condition among the workers is good and no harmful effect was seen. And of course it is not true..*Trying to under stand HWE was not easy since there was a lot of explaination and debate about its nature, a good example could be seen in Choi 1992 when he described the differences in opion regarding HWE. The article included the openion of nine different participents familiar with HWE. *The participnt had different openion about the core of the problem*Some of them considered HWE as a source of selection bias, others considered it as a confounding . A third group considered it as a mixture of both. While others considered it as a coparison problem**According to this difinition is HWE is a source of selection bias. McMichael considered it a source of selection bias because the characteristic of the occupational cohort selected to participate in a work are differ they are healthy-from the general population which contains healthy and unhealthy casescause only healthy workers will gain and remain employed.*The selection bias occurred from the initial choosing of workers (mainly healthy workers) and the factors that influence the contineouty of work such as leaving thework because of sickness.**An example on confounding is the following diagram. The confounder should associate with both exposure and outcome*According to Monson HWE is considered as a case of confounding. The confounder is the unmeasured health status. also He considered the appearance of selection bias depends on the action of the investigators, whereas confounding is a characteristic of the study population.*Going back to our example GHS is associated with exposure (Employment in industry and associated with outcome death