Cover Image

PAPERBACK
$49.00



View/Hide Left Panel

"shrinking" the Yi toward the mean . The estimators of µi have the correct dispersion in that

In fitting the Weibull model in (A.1), we found that the estimate of the variance of the log10TD*50 based on (A.9) appeared to be excessively large in a small number of cases. In order not to underestimate the between-study variability based on (B.2), we used a trimmed mean S*si2/n*, in which the largest and smallest 10% of the observed values of si2 were omitted (Hampel et al., 1986, p. 79). Specifically, the summation S* covers only those n* = 153 observations falling in the central 80% of the distribution of the si2.

Annex C: Adjustment of Potency Values for Less than Lifetime Exposure

In order to ensure that TD50 values for different chemicals are comparable, some adjustment for differences in the duration of the experimental period is desirable. Gold et al. (1984) adjusted TD50 values by a multiplicative factor of f2, where f represents the fraction of a two year period encompassed by the study period. This effectively scales the TD50 values to a standard two year rodent lifetime. Specifically, we have

where the TD50 denotes the estimate of carcinogenic potency based on the observed data for the actual experimental period, and denotes the standardized value.

To motivate the use of the adjustment factor f2, consider the extended Weibull model



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 164
APPENDIX F 164 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. "shrinking" the Yi toward the mean . The estimators of µ i have the correct dispersion in that In fitting the Weibull model in (A.1), we found that the estimate of the variance of the log10TD*50 based on (A.9) appeared to be excessively large in a small number of cases. In order not to underestimate the between-study variability based on (B.2), we used a trimmed mean Σ*σi2/n*, in which the largest and smallest 10% of the observed values of σi2 were omitted (Hampel et al., 1986, p. 79). Specifically, the summation Σ* covers only those n* = 153 observations falling in the central 80% of the distribution of the σi2. ANNEX C: ADJUSTMENT OF POTENCY VALUES FOR LESS THAN LIFETIME EXPOSURE In order to ensure that TD50 values for different chemicals are comparable, some adjustment for differences in the duration of the experimental period is desirable. Gold et al. (1984) adjusted TD50 values by a multiplicative factor of f2, where f represents the fraction of a two year period encompassed by the study period. This effectively scales the TD50 values to a standard two year rodent lifetime. Specifically, we have where the TD50 denotes the estimate of carcinogenic potency based on the observed data for the actual experimental period, and denotes the standardized value. To motivate the use of the adjustment factor f2, consider the extended Weibull model

OCR for page 164
APPENDIX F 165 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. depending on both dose d and time t. Under this model, the TD50(t) evaluated at time t is given by Thus, the ratio of TD50's at two distinct times t1 and t2 is where f = t1/t2. In the CPDB, Gold et al. (1984) use a one-stage model with k = 1 and set p = 2 based on empirical observations reported by Peto et al. (1984), leading to their adjustment factor f2. In our applications of the Weibull model in (A.1), we will use a similar adjustment factor of f2/k to standardize TD50 values to a two year rodent lifetime. For a multi-stage model of the form allowing for the effects of both dose d and time t, the TD50 at time t is obtained as the solution of the equation It follows that the standardized value of the TD50 is obtained as the solution of the equation As with the Weibull model, we set p = 2 in the applications considered in this paper.