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HTDS Guide > Findings and
Interpretations > How Reliable Are the Results of the HTDS?
HTDS Guide
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How Reliable Are the Results of the HTDS?
An important consideration in any epidemiological study is the
reliability of the information. In the case of HTDS, researchers could
be confident that their assessments of thyroid disease were very
reliable because participants received thorough examinations of their
thyroid glands by experienced doctors.
On the other hand, estimating radiation doses to the participants'
thyroid glands was inherently uncertain. This is because their doses
depended not only on where they lived, but also on the quantities and
sources of the food and milk products they consumed when they were
infants and young children during 1944 through 1957. Highly accurate
recollection of information from so long ago is simply not possible. In
addition, the amounts of iodine-131 released from Hanford, and how that
iodine-131 moved through the environment, are not known exactly.
In order to account for the effects of this uncertainty about the
participants' doses, HTDS researchers performed special statistical
analyses. They also examined whether risks of thyroid diseases and
thyroid ultrasound abnormalities differed according to where the
participants lived and/or how much milk they consumed, rather than their
estimated doses. None of these additional analyses led to a conclusion
that risks of thyroid disease increased with increasing exposure to
Hanford's iodine-131.
No epidemiological study can state with absolute certainty whether or
not an exposure (such as Hanford's iodine-131) has affected people's
health. Nevertheless, studies such as the HTDS can be conducted in ways,
and with enough participants, to provide confidence that if an
association exists it is likely small. In the HTDS, scientists did not
observe an association using a careful study design, thorough clinical
evaluation, and several alternative approaches for estimating radiation
dose and analyzing study data.
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