Cox J L, Heyse J F, Tukey J W
Pharmaceutical Research and Development, Merial Limited, 2100 Ronson Road, Iselin, New Jersey 08830-3077, USA.
Exp Parasitol. 2000 Sep;96(1):1-8. doi: 10.1006/expr.2000.4550.
Because of the positive skewness of parasite distributions and the greater constancy of percentage of response of therapy in animal populations, parasite count data are conventionally transformed logarithmically before combining results from different animals, either all controls or all treated. Observations of zero counts raise difficulties, since the logarithm of zero is not useful. In this study, several types of zero count adjustments are compared. Two systems for assigning values to zero counts were considered: a fixed system, which assigns the same value to all zero counts regardless of the proportion of such counts in a treatment group, and a variable system, which replaces zero counts with a value based on the proportion of zero counts in the group. The values assigned by either system are then adjusted to reflect aliquot size. An evaluation was performed by using 32 compound Poisson lognormal distributions, three sample sizes, and three representatives of each zero count adjustment system. The Poisson lognormal distribution provides a convenient method with which to provide variability greater than Poisson. Expected values of the sample estimate of the (known) population mean were calculated for each of the 576 combinations of these factors, and the bias associated with each combination was derived. The bias associated with the three representatives of the variable adjustment system was similar. The variable adjustment system had a lower overall bias than any representatives of the fixed adjustment system.
由于寄生虫分布呈正偏态，且动物群体中治疗反应百分比的稳定性更高，因此在合并来自不同动物（无论是所有对照组还是所有治疗组）的结果之前，通常会对寄生虫计数数据进行对数转换。零计数的观测会带来困难，因为零的对数没有意义。在本研究中，对几种类型的零计数调整方法进行了比较。考虑了两种为零计数赋值的系统：一种是固定系统，无论治疗组中此类计数的比例如何，都为所有零计数赋予相同的值；另一种是可变系统，根据组中零计数的比例用一个值替换零计数。然后对这两种系统赋予的值进行调整，以反映等分试样大小。使用32种复合泊松对数正态分布、三种样本量以及每种零计数调整系统的三个代表进行了评估。泊松对数正态分布提供了一种方便的方法，可用于提供比泊松分布更大的变异性。针对这些因素的576种组合中的每一种，计算了（已知）总体均值的样本估计的期望值，并得出了与每种组合相关的偏差。可变调整系统的三个代表相关的偏差相似。可变调整系统的总体偏差低于固定调整系统的任何代表。
