Revival: Biological Effects of Low Level Exposures to Chemical and Radiation (1992)

134 BIOLOGICAL EFFECTS OF LOW LEVEL EXPOSURES

zero, the lowest value possible, is not unusual since zero is the lower normal
limit of a 95% normal range (Table 7.5A).
Tables 7.6 and 7.7 contain sensitivity probabilities for detecting hormetic
effects in SR studies. These depend on the relative risk (RR), defined in the
present context by

RR = (true expected number)/(calculated expected number)

If the null hypothesis of no difference in risk is true, then the RR is unity. In
such case (the last columns in Tables 7.6 and 7.7), the actual effect is
neutral, and the corresponding tabular entries are the probabilities of Type
I errors. These cannot exceed 2.5%, and are often considerably below 2.5%
because of the discreteness of the Poisson distribution. For all the other
columns, where the RR is lower than one, there is a hypothesized beneficial
effect relative to the standard population, and the tabular entries are the
probabilities of TLs for the corresponding hypothetical RRs. These sensitiv­
ity probabilities tend to increase as the expected cases increase (though in an
erratic manner due to discreteness) and do increase systematically as the RR
decreases.

Meta-Analytic Techniques

Sometimes there may be several studies available with low-level exposures

to a substance, yet each individual study has insufficient sensitivity to detect

a hormetic effect. However, when the studies are pooled or viewed collec­

tively, a clear pattern may emerge. Meta-analysis is a collection of statistical

techniques for objectively synthesizing or pooling similar studies. Meta-

analytic results are not always definitive. An attempt to synthesize the

literature on effects of passive smoking via meta-analytic techniques yielded

inconclusive results.11

There are numerous philosophical and practical problems involved in

meta-analysis. Many of these stem from dissimilarities between the several

studies being synthesized. No two studies are exact duplicates of each other,

and it may be impossible to statistically adjust for differences between them

in a satisfactory way. Meta-analysis has been described by some as contro­

versial and by others as the wave of the future.12 Like any statistical tech­

nique, it can be misused. A list of problems and questions concerning meta­

analysis has been compiled by Spitzer.13

Meta-analytic studies of hormetic effects can be expected to suffer from

“publication bias” —nondetrimental results tend not to be submitted for

publication by investigators, and such nondetrimental results that are sub­

mitted tend to be rejected by editors. Publication bias could result in signifi­

cant overestimates of risk in humans and be detrimental to rational health

policy decisions. A group of epidemiologists recently concluded that publi­

cation bias was a definite problem in their field.114 Nevertheless meta-