在横断面数据进行相关危险因素（连续性变量）和疾病（二分类变量）的相关性分析中，怎么才能知道自己得样本量够不够呢？下面我搜索到的类似的文献，求助有人知道这是具体怎么样计算出来的么？    Sample size was estimated based on the prevalence (13·7 %) of   the MetS in China in 2005(4) and the reported OR (0·76) of the   MetS for the highest (v. lowest) quartileof serum carotenoids(20).   In each group, 537 subjects would provide 80 % power to detect   the association (OR 0·76) at the significance of 0·05