Dear all:
 
Afer estimating ARIMA, It is convenient to test if residual is white noise in Gretl.
But I have two questions:
1. Gretl provide Ljung-Box Q statistic, residual acf and pacf.
   Some paper test lag of 1 to 24,
   Some only test lag of 6, 12, 18, 24.
   Some only test lag of 1, 12, 24. 
I am confused about the lag of Q statistic, acf and pacf.
 
2. Some paper test ARIMA residual by Q statistic, residual acf and pacf.
   But some just test by  Q statistic. Ignoring residual acf and pacf.
  Are residual acf and pacf necessary when testing ARIMA residual ?
 
Thanks a lot