Difference between arma and arima model?

Answer 1

An ARMA model contains parts for an AR and MA model so is ARMA(p,q). An ARIMA model is extended as it includes the extra part for differncing. If a dataset exhibits long term variation (i.e. trend-cycle componenet) the ACF graph will show a straight line edge and will not quickly drop to zero. In this case it is useful to difference the data. This simply takes each datapoint and calculates the change from the previous datapoint. The ARIMA model is ARIMA(p,d,q) where p is the order of the AR part, d is the number of times differncing has been carried out and q is the order of the MA part. The extension allows the model to deal with long term variation better so improves the uesfulness of this modelling technique