Activity Number:
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317
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Type:
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Contributed
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Date/Time:
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Wednesday, August 14, 2002 : 10:30 AM to 12:20 PM
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Sponsor:
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Biometrics Section*
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Abstract - #300451 |
Title:
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Critical Values and Power for a Test of Group Differences in Rodent Carcinogenicity Bioassays Based on the Beta-Binomial Distribution with Small Samples
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Author(s):
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Ben Ahlstrom*+ and John Lawson+
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Affiliation(s):
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Bandag Inc. and Brigham Young University
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Address:
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6500 49th Street, Muscatine, IA, 52761, USA 223B TMCB BYU, Provo, UT, 84602, USA
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Keywords:
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likelihood ratio test ; rodent carcinogenicity bioassay ; two control groups ; Fisher's exact test ; overdispersion ; significance level
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Abstract:
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In this paper we show that false positives are possible when analyzing dichotomous data from rodent carcinogenicity bioassays using Fisher's exact test. The false positives occur when there are multiple control groups in a study and overdispersion is present. When comparing tumor counts from two control groups to one treated group in a rodent bioassay, 3×2 tables of counts are analyzed. Under various overdispersion scenarios (represented by combinations of beta-binomial parameters), we found 7-85% false positives by enumerating the distribution of Fisher's exact test. To reduce false positives, in the analysis of 3×2 tables, we propose the use of a likelihood ratio test for treatment-effect based on the beta-binomial distribution. Small sample sizes in rodent bioassays do not justify an asymptotic Chi-square distribution of the statistic. Therefore, critical values for the proposed test statistic were found by enumerating its distribution, and a small Monte Carlo Simulation was used to study its power. The simulation results indicate this likelihood ratio statistic controls the 0.05 significance level between .040 and .070 and that it has desirable power properties.
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