A mixture of selection bias and confounding (Checkoway et al 1989, Li & Sung 1999) Mixture of biases?

A Exposed What is the ideal reference? t0t0 t1t1

A Exposed t0t0 t1t1 A Unexposed t1t1 t0t0 What is the ideal reference?

(Miettinen 1988) HWE is a comparison problem

Components of HWE

Healthy hire effect (Arrighi & Hertz-Picciotto 1994, Checkoway et al 1989)

Healthy worker survivor effect HWE occurs because of differential survival of workers with different risk of disease at work with the same exposure (Arrighi & Hertz-Picciotto 1994, Checkoway et al 1989)

The length of time the population has been followed (time-since-hire) HWE is strongest during active employment and rapidly disappear following the cessation of employment.

Monson (1986) divided the follow- up into two phases; a dynamic phase and a stable plateau

Beneficial effect of work Improved access to health care Routine disease screening Physical exercise (Doll 1988)

Factors affecting HWE

Time related factors Age at hire Age at risk Duration of employment

Age at hire Workers with high age at hire will show strong HWE: RR for all causes of mortality, according to Fox & Collier (1976) (Checkoway et al 1989) Age at hire

Age at risk Workers with high age at risk will show low HWE: RR for all causes of mortality by age at risk (Checkoway et al 1989) Study Age at risk 75 Fox and Collier(1976) McMichael et al (1976) Delzell and Monson (1981)

Age at risk (McMichael 1974)

Duration of employment The decrease in mortality is more obvious in the longest employment duration categories (Baillargeon & Wilkinson 1999)

Disease entity

Socioeconomic status The HWE is stronger in more qualified jobs (Axelson 1988) Professional workers demonstrate a stronger overall HWE (Baillargeon & Wilkinson 1999)

Gender The effect is stronger for females than males (Howe 1988)

Avoiding HWE

Identify the correct external comparison group, or a representative sample of it (McMichael 1987)

A number of Authors have suggested to assemble a large group of employed persons and use their mortality rates as a standard basis for external comparison (Monson 1986)

Progress from external comparisons to internal comparisons (McMichael 1987)

Start the study after a latency time (lag) (McMichael 1987)

Trace and include those individuals who leave the workforce before retirement (McMichael 1987)

A Exposed t0t0 t1t1 A Unexposed t1t1 t0t0 Conclusion: the core of the HWE is lack of an ideal reference

Summary & Conclusion HWE is caused by inadequate reference group (i.e., a comparison problem) (Miettinen 1988) HWE is a complex and problem creating bias, it comprises several factors, and may be modified by a number of factors (McMichael 1974) It is not possible to make generalizations in a particular case of HWE (Geoffrey 1989)