Folkes, Here is the answer from Andrew Karp..... Direct link
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Allow me to weigh in on this topic. It comes up alot when I give SAS training classes. First, RUN and QUIT are both "explicit step boundaries" in the SAS Programming Language. PROC and DATA are "implied step boundaries."
Example 1: Two explicit step boundaries.
DATA NEW;
SET OLD:
C = A + B;
RUN;
PROC PRINT DATA=NEW;
RUN;
In this example, both the data and the proc steps are explicitly "ended" by their respective RUN statements.
Example 2: No explicit step boundaries.
DATA NEW;
SET OLD;
C = A + B;
PROC PRINT DATA=NEW;
In this example, the data step is implicitly terminated by the PROC statement. But, there is no step boundary for the PROC PRINT step/task, so it will not terminate unless/until the SAS supervisor "receives" a step boundary.
Some PROCS support what is called RUN group processing. These include PROCs DATASETS and PLOT in the BASE Module, PROCs REG and GLM in the STAT module and ARIMA in the ETS module.
Example 3: PROC DATASETS with RUN group processing.
PROC DATASETS LIBRARY = MYLIB;
MODIFY SET1;
FORMAT VAR1 DOLLAR12.2;
LABEL VAR1 "Dollars Paid";
RUN;
DELETE SET2;
RUN;
CHANGE SET3 = SET2;
RUN;
QUIT;
In this example, three separate data mangement tasks are carried out in the order they were written/coded. First a label and format are added to the descriptor portion of SET1, then SET 2 is deleted, and then SET3 is renamed to SET2. The RUN ends each command in the PROC DATASETS step (MODIFY, DELETE, CHANGE) and QUIT ends the step. If the explicit step boundary had been omitted, the step would have been implicity terminated by a subsequent PROC or DATA statement. If there were no implied step boundary following the last RUN command then the PROC DATASETS step would not terminate.
The same holds true with other RUN-group enabled PROCs in SAS Software. The ARIMA procedure in the ETS module, for example, implements what is called the Box-Jenkins methodology to analyze a time series and then generate future forecasted values from the existing series. There are three parts to this methodology, which are implmented in PROC ARIMA using (in this order), the IDENTIFY, ESTIMATE and FORECAST statements. The output from each statement is needed by PROC ARIMA to move to the next step in the process, and an experienced forecaster can look at the output generated by the IDENTIFY statement and then write the appropriate ESTIMATE statement syntax, and then do the same thing with the output generated by the ESTIMATE statement to write the proper FORECAST statement syntax. Once the analyst is satisfied with their model, they can terminate the PROC ARIMA step with a QUIT statement and move on to the next part of their project.
I hope this has been helpful.
Andrew Karp
Sierra Information Services
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