The within-year distribution of 15 positive sampling events1 yielding 36 positive pooled urine samples from July 2008 to June 2011.

1Five sampling events yielding positive samples (and three sampling events yielding negative samples) were ‘follow-up’ events prompted by previous detections, and are excluded as these data are not independent.1. 5例获取阳性样本的采样事件（另有3例获取阴性样本的采样事件）为受此前检测结果触发的后续采样事件，因此类数据不具备独立性，故将其排除。