变量抽取 sample 10,count//简单随机抽样，抽取10个观测值 sample 10 if female==1,count//在女性数据中抽取10个样本并保留所有男性样本 sample 10 ,count by(female)//在男性和女性样本中各抽取10个样本 sample 5 ,count by(female married)//对female和married的各种组合各抽取5个样本
