Campbell M J, Machin D, D'Arcangues C
University of Southampton, Southampton General Hospital, U.K.
Stat Med. 1991 Feb;10(2):241-54. doi: 10.1002/sim.4780100208.
The vaginal ring is one method of contraception currently under investigation and development by the Human Reproduction Programme of WHO. One factor that reduces its acceptability is that some women experience involuntary expulsions of the ring. WHO have collected prospective data on 1005 women from 19 centres worldwide to investigate various aspects of ring use including factors that might affect expulsions. For each subject the data include number of expulsions, age, parity, ponderal index, number of days of ring use and health care centre. To model the factors influencing expulsion, the number of expulsions was first considered as a Poisson variate with mean directly proportional to the log of the duration of ring use. Such a model gave a poor description of the data and it appeared that the number of expulsions was considerably over-dispersed for an assumed Poisson variate. This paper examines a variety of strategies for coping with the resulting lack of fit of the model. It turns out that a truncated Poisson model gives a better description of the expulsion data and that health care centre and parity influence the rates.
阴道环是世界卫生组织人类生殖计划目前正在研究和开发的一种避孕方法。降低其可接受性的一个因素是一些女性会出现阴道环的意外脱落。世界卫生组织收集了来自全球19个中心的1005名女性的前瞻性数据，以调查使用阴道环的各个方面，包括可能影响脱落的因素。对于每个受试者，数据包括脱落次数、年龄、产次、体重指数、使用阴道环的天数以及医疗保健中心。为了建立影响脱落的因素模型，首先将脱落次数视为泊松变量，其均值与使用阴道环持续时间的对数成正比。这样的模型对数据的描述很差，而且对于假定的泊松变量，脱落次数似乎过度离散。本文研究了多种应对模型拟合不足问题的策略。结果表明，截断泊松模型对脱落数据的描述更好，而且医疗保健中心和产次会影响脱落率。
