How to Exclude Lower Order Lags from a Custom ARIMA Model in SPSS Forecasting Module

Question

I'm trying to use an ARIMA model to generate forecasts using monthly revenue data that exhibits both trend and seasonality.

I've used the Expert Modeler from the SPSS Forecasting Module to determine that an

ARIMA (3, 1, 0)(0,1, 0)

best fits my data. I've noticed that the Expert Modeler only includes an AR term at Lag 3 because the lower orders are not statistically significant.

I would like to fit a custom ARIMA that mimics the Expert one as closely as possible so that I can specify a Level Shift Outlier that the Expert Modeler does not pick up (p-value = .012). It is important that I include this outlier because it:

1. Makes sense intuitively
2. Improves the fit of the model drastically

However, I can't seem to find a way to specify a custom ARIMA Model that excludes the statistically insignificant lower order AR terms, or a way to manually specify outliers in the Expert Modeler.

I was wondering if there is an option to handle this in SPSS? Or if it is something that I should even be concerned about?

Answer 1

you have to open a syntax windows and change the parameters manually.

Create and run your model with all lags. Then, copy the TSMODEL data tha your output shows before your model. Usually, it comes like this:

PREDICT THRU YEAR 2000 QUARTER 4. 
* Time Series Modeler. 
TSMODEL 
   /MODELSUMMARY  PRINT=[NONE] 
   /MODELSTATISTICS  DISPLAY=YES MODELFIT=[ SRSQUARE RSQUARE] 
   /MODELDETAILS  PRINT=[ PARAMETERS RESIDACF RESIDPACF FORECASTS]  PLOT=[ RESIDACF RESIDPACF] 
   /SERIESPLOT OBSERVED FORECAST FIT 
   /OUTPUTFILTER DISPLAY=ALLMODELS 
   /AUXILIARY  CILEVEL=95 MAXACFLAGS=24 
   /MISSING USERMISSING=EXCLUDE 
   /MODEL DEPENDENT=GDP 
      PREFIX='Model' 
   /ARIMA AR=[12,11,10,9,8,7,6,5,4,3,2,1]  DIFF=1  MA=[0]  ARSEASONAL=[0]  DIFFSEASONAL=0  MASEASONAL=[0] 
      TRANSFORM=NONE  CONSTANT=YES 
   /AUTOOUTLIER DETECT=OFF.

After that, go to FILE > NEW > SYNTAX and paste what you have copied. Now, you only have to change the AR or MA parameters:

AR=[12,11,10,9,8,7,6,5,4,3,2,1] for AR=[8,4,2,1] and etc.