请问P 为什么等于2

问题如下：

Question For an initial regression model with five independent variables and a sample size of 100, a Breusch–Godfrey test is carried out to test for serial correlation in the model's residuals up to lag two. The degrees of freedom of the F-statistic for this test are:

选项：

A.A.92 and 2. B.B.93 and 2. C.C.94 and 2.

解释：

• A is Correct because, for a Breusch–Godfrey test, the test statistic is approximately F-distributed with n – p – k – 1 and p degrees of freedom, where p is the number of lags, n is the sample size, and k is the number of independent variables.

  Here, n = 100, p = 2, and k = 5, so n – p – k – 1 = 100 – 2 – 5 – 1 = 92, hence the degrees of freedom are 92 (= n – p – k – 1) and 2 (= p).

• B is Incorrect because the solution omits subtracting 1 in the first number of degrees of freedom, i.e., n – p – k = 100 – 2 – 5 = 93.

• C is Incorrect because the solution adds 1 instead of subtracting 1 in the first number of degrees of freedom, i.e., n – p – k + 1 = 100 – 2 – 5 + 1 = 94.