Solved – How to interpret ACF and PACF

arimaautocorrelationtime series

ACF PACF

ACF and PACF after differencing time series

Can you please tell me what should be the values of ACF and PACF from the graphs I have attached? I think it should be ($p=0$, $d=1$, $q=3$). I have differenced the data once so $d=1$ and there are three spikes in ACF so $q=3$. But function auto.arima from "forecast" package in R is giving the answer as (0,0,1).

Also, what should be the values for the seasonal $P$, $D$, $Q$ and its period value?

Best Answer

Firstly, inferring from the ACF and PACF plots of the data, I would say your series is already stationary. There is no need for first order differencing.

If the lag-1 autocorrelation is more negative than -0.5 (and theoretically a negative lag-1 autocorrelation should never be greater than 0.5 in magnitude), this may mean the series has been overdifferenced.

Please refer to the following link for a better understanding on selecting the degree of differencing and order of AR and MA for ARIMA models.


https://people.duke.edu/~rnau/411arim3.htm