偏倚(bias)是在研究组间变量差异时产生的一种系统误差。临床科研的结论常通过2组的比较而获得,如所获得2组间结果的差异并未代表真实情况,而是由于选择研究对象和进行研究、收集资料过程中人为产生的,从而歪曲了研究结果,这种现象称为偏倚.临床研究中的偏倚繁多,大致可分为3类。(1)选择性偏倚:指选择研究对象和对照组时,抽取样本的方法有错,造成样本的有关变量不能代表总体。如 Bias (bias) is a systematic error that arises when studying differences in variables between groups. The conclusion of clinical research is usually obtained by comparing two groups. If the differences between the two groups obtained do not represent the real situation, they are distorted because of the selection of subjects and research, and the artificial collection of data during the research As a result, this phenomenon is called biases, and there are many biases in clinical research and can be broadly categorized into three categories. (1) selective bias: refers to the selection of subjects and control groups, the method of sampling is wrong, resulting in sample variables can not represent the overall. Such as