这不就是预测你的Y的嘛

其实这个命令跟predict Yhat得出的结果是一样的，只不过带上xb是用来区分resid的，带上xb表示得出的y值，而带上resid得出的是残差值，以此来进行说明，也更容易记清与比较。

感谢！

    xb calculates the linear prediction from the fitted model.  That is, all
        models can be thought of as estimating a set of parameters b1, b2,
        ..., bk, and the linear prediction is y = xb.  For linear regression,
        the values y are called the predicted values or, for out-of-sample
        predictions, the forecast.  For logit and probit, for example, y is
        called the logit or probit index.

        x1, x2, ..., xk are obtained from the data currently in memory and do
        not necessarily correspond to the data on the independent variables
        used to fit the model (obtaining the b1, b2, ..., bk).

y=xb+r