大神请教个问题，作双重差分倾向得分匹配，参考的陈强老师的书，用的diff命令，结果显示
KERNEL PROPENSITY SCORE MATCHING DIFFERENCE-IN-DIFFERENCES
    Estimation on common support
    Report - Propensity score estimation with probit command
    Atention: _pscore is estimated at baseline

outcome does not vary; remember:
                                  0 = negative outcome,
        all other nonmissing values = positive outcome
r(2000);

是什么原因呀？