Dobson R J, Sangster N C, Besier R B, Woodgate R G
Murdoch University, School of Veterinary & Biomedical Sciences, South Street Murdoch, WA 6150 Australia.
Vet Parasitol. 2009 Apr 6;161(1-2):162-7. doi: 10.1016/j.vetpar.2008.12.007. Epub 2008 Dec 13.
The process of conducting a faecal egg count reduction test was simulated to examine whether arithmetic or geometric means offer the best estimate of efficacy in a situation where the true efficacy is known. Two components of sample variation were simulated: selecting hosts from the general population which was modelled by the negative binomial distribution (NBD), and taking an aliquot of faeces from the selected host to estimate the worm egg count by assuming a Poisson distribution of sample counts. Geometric mean counts were determined by adding a constant (C) to each count prior to log transformation, C was set at 25, 12 or 1. Ten thousand Monte Carlo simulations were run to estimate mean efficacy, the 2.5% (lower) and the 97.5% (upper) percentile based on arithmetic or geometric means. Arithmetic means best estimated efficacy for all different levels of worm aggregation. For moderate levels of aggregation and with C=1 the geometric mean substantially overestimated efficacy. The bias was reduced if C was increased to 25 but the results were no better than those based on arithmetic means. For very high levels of aggregation (over-dispersed populations) the geometric mean underestimated efficacy regardless of the size of C. It is recommended that the guidelines on anthelmintic resistance be revised to advocate the use of arithmetic means to estimate efficacy.
模拟了进行粪便虫卵计数减少试验的过程，以检验在已知真实疗效的情况下，算术平均值或几何平均值是否能提供对疗效的最佳估计。模拟了样本变异的两个组成部分：从以负二项分布（NBD）建模的总体中选择宿主，以及从选定的宿主中取一份粪便样本，通过假设样本计数服从泊松分布来估计虫卵计数。在对数转换前给每个计数加上一个常数（C）来确定几何平均计数，C设置为25、12或1。进行了一万次蒙特卡洛模拟，以估计基于算术平均值或几何平均值的平均疗效、2.5%（下限）和97.5%（上限）百分位数。对于所有不同程度的蠕虫聚集，算术平均值对疗效的估计最佳。对于中等聚集程度且C = 1时，几何平均值大大高估了疗效。如果将C增加到25，偏差会减小，但结果并不比基于算术平均值的结果更好。对于非常高的聚集程度（过度分散的群体），无论C的大小如何，几何平均值都会低估疗效。建议修订抗蠕虫药耐药性指南，提倡使用算术平均值来估计疗效。
