McMahon A D, Evans J M, McGilchrist M M, McDevitt D G, MacDonald T M
Department of Clinical Pharmacology, University of Dundee, Scotland, UK.
Pharmacoepidemiol Drug Saf. 1998 Jul;7(4):275-80. doi: 10.1002/(SICI)1099-1557(199807/08)7:4<275::AID-PDS363>3.0.CO;2-N.
To determine the appropriate size of risk windows in both exposed and unexposed sub-cohorts.
Data was taken from a previous study of upper gastrointestinal haemorrhage and perforation. The length of each prescription for NSAIDs was estimated. The risk was calculated for the duration of a prescription plus increments of -30, -25,..., +115, +120 (i.e. 31 increments). Ten unexposed groups were re-sampled for each increment (stratified for age and sex), using the same lengths of risk window as the exposed group. Mean risks and rate-ratios were calculated (per thousand person-years).
The NSAID risk rose from 3.52 at -30 days to a peak of 5.82 at -15 days, and then decreased gradually to 2.83 at +120 days. Unexposed risk was variable for the negative increments, and decreased gradually from 2.16 at +0 days to 1.54 at +120 days. The rate-ratio rose from 1.55 at -30 days to a peak of 2.85 at -5 days, and then decreased to 1.85 at +120 days.
Risk windows should be the same as (or slightly less than) the calculated length of a prescription. Lengthy windows should not be used for unexposed comparator groups (the exposed windows may be randomly allocated).
确定暴露和未暴露亚队列中风险窗口的合适大小。
数据取自先前一项关于上消化道出血和穿孔的研究。估算每种非甾体抗炎药（NSAIDs）处方的时长。计算处方时长加上 -30、-25、...、+115、+120（即31个增量）期间的风险。对每个增量，重新抽取10个未暴露组样本（按年龄和性别分层），使用与暴露组相同长度的风险窗口。计算平均风险和率比（每千人年）。
NSAIDs风险从 -30天的3.52升至 -15天的峰值5.82，然后逐渐降至 +120天的2.83。未暴露组的风险在负增量时变化不定，从 +0天的2.16逐渐降至 +120天的1.54。率比从 -30天的1.55升至 -5天的峰值2.85，然后降至 +120天的1.85。
风险窗口应与处方计算时长相同（或略短）。不应为未暴露的对照队列使用过长的窗口（暴露窗口可随机分配）。
