were asked for clarification. The results from these
studies support the results reported in this review.
The searching in multiple databases and websites of
major health and safety institutions makes it very likely
that most of the published studies have been located.
However, publication bias due to omission of nonpublished
negative studies is still conceivable. On the other
hand, review of the excluded lower-quality studies
revealed that there were also relatively few studies with
a reported statistically negative outcome. Therefore, it
was assumed that the risk of publication bias was low