Review of the Hanford Thyroid Disease Study Draft Final Report (2000) / Chapter Skim
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5 Evaluation of Statistical Data Analysis
5 Evaluation of Statistical Data Analysis
Pages 86-98
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From page 86... ...
for probabilities is common in radiation epidemiology and is generally based on biologic or radiation-protection considerations. Such models, however, can present difficulties in estimation, in that negative probabilities are not allowed to occur but can appear during the iterative procedure used to produce maximum likelihood estimates, especially if a generally negative dose-response relationship is evident in the data.
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From page 87... ...
In the HTDS Draft Final Report, there was an overreliance on the maximum likelihood fitting of the linear doseresponse model. For several of the important outcome variables, such as thyroid carcinoma, the mode!
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From page 88... ...
Similarly, one expects radiation-epidemiology reports to include tables that show observed and expected numbers of disease outcomes according to dose groups; these key tables were absent from the Draft Final Report.
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From page 89... ...
. There is sizable uncertainty in the doses reconstructed for individuals based on residential and especially dietary histories, and variations related to source term, meteorologic uncertainties, pasture deposition, milk concentrations of }3~{, source of milk, and
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From page 90... ...
The statistical-methods section (section VTT) of the Draft Final Report described a mode!
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From page 91... ...
ANALYSIS OF POTENTIAL CONFOUNDING OR EFFECT-MODIFYING VARIABLES it was not clear from the Draft Final Report how confounders of dose-response relationships were treated, and results adjusted for possible confounders were rarely given. The HTDS investigators conducted analyses of the venous thyroiddisease end points to evaluate a number of possible risk factors for confounding effects or effect modification (section VIlI.D.20)
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From page 92... ...
Likewise, because the magnitude of thyroid doses from AT fallout from the NTS and from global fallout was not greatly different from the Hanford doses in many study subjects, tables showing the results of analyses stratified by magnitude of NTS or global fallout are potentially important. OUT-OF-AREA ANALYSES The HTDS investigators took care to examine the results for the out-of-area participants, those who proved never to have been in the dosimetry area during the time of 13~{ exposure.
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From page 93... ...
GEOSTRATUM VERSUS. DISEASE The HTDS investigators examined thyroid morbidity according to geographic areas, which they called "geostrata".
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From page 94... ...
it is recommended that the Hanford investigators perform a similar type of analysis to examine the possible association of thyroid nodules and other thyroid diseases with 13lI dose. This would provide assurance that a possible confounding variable had been sufficiently evaluated, either to ensure that a positive association was not masked by the geostratum variations or to detect a masked association.
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From page 95... ...
Persons living in various geographic areas might vary in their baseline risk of thyroid diseases because of differences in dietary iodine intake and other unknown factors. Perhaps more important, the rates of detected disease are based on examinations and depend on the methods and criteria of the examinations; this produces screening effects that cannot be readily disentangled to make meaningffi} comparisons with disease-rates from other geographic regions that did not have comparable screening.
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From page 96... ...
The HTDS Draft Final Report does not indicate any attempt to compare the HTDS thyroid-nodule prevalence with that found in unirradiated populations. Reported prevalence rates in unirradiated groups are available from about a dozen studies in the literature, so, in principle, it is possible to do, although again there would be a question about comparability with respect to screening intensity.
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From page 97... ...
A table showing mean doses by amount of milk consumption in a given geo stratum would be illuminating in indicating the degree to which dose variations were driven by milk consumption versus geographic location. That is important for understanding the degree to which the study's negative results might have occurred because of lack of reliability or validity in the reported milk-consumption rate.
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From page 98... ...
A useful analysis would examine associations using only those with interview information so as to yield results that minimize dose misclassification. Section 6 of this report describes the effect of milk-consumption measurement error on the statistical power of the study.
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