For the first case: it is enough to select cases from 1 to 80 to build the model, and ... time interval or cases", then on "Interval": write 1 in the first box and 80 in the.
Supposing you have a database of 100 time points (effective dates, not future dates). What is your objective ? 1) do you desire to simulate what would have happened between the time interval 81-100 (that is, 20 actual dates), in a alternative situation respect the real one ? 2) or do you desire to predict what will happen between the time interval 101-120 (that is, 20 future dates) ? In the "researcher language" the first case is a simulation, and the second case is a forecast. In the "spss language" both cases are a forecast. How to distinguish them using the same SPSS function named "Forecast" ? For the first case: it is enough to select cases from 1 to 80 to build the model, and reserve the cases for 81 to 100 for the "forecast" (that actually is a simulation). For the second case: the same procedure, but it is necessary to extend the database to 120 time points, adding hypothetical values; thus select cases from 1 to 100 to build the model, and reserve the cases from 101 to 120 for the forecast (that is a real forecast). What is the utility of the first case ? It is possible to simulate what would have happened in presence of alternative values of the exogenous variables. I think that you are interested to the first case. If not, you can easily shift to the second case applying the same procedures described in this and in the other two files, simply extending the lenght of your databases to cover the future periods. The only difference is: 1) first case - for the period 81-100 you have two set of values for the exogenous: one is actual and the other is hypothetical 2) second case - for the period 101-120 you have again two set of values for the exogenous, but both are hypothetical
Click on "Data", then on "Select cases" Click on "Based on time interval or cases", then on "Interval": write 1 in the first box and 80 in the second. Close the window (second case: 100 in the second box).
