Table 4

Potential forms of bias and the direction that they may have influenced the results.

Cause or Source of Bias

Direction of the Effect

Accuracy of "Usual Occupation" on Death Certificate

Either (most likely toward the null)

Only deceased individuals eligible as controls

Toward the null

Lack of specific job titles

Toward the null

Migration into and out of region

Toward the null

No information on length of employment

Toward the null

No information on specific chemicals used

Toward the null

Forand Environmental Health: A Global Access Science Source 2004 3:7   doi:10.1186/1476-069X-3-7