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The DTREE Procedure

Introductory Example

A decision problem for an oil wildcatter illustrates the use of the DTREE procedure. The oil wildcatter must decide whether or not to drill at a given site before his option expires. He is uncertain about many things: the cost of drilling, the extent of the oil or gas deposits at the site, and so on. Based on the reports of his technical staff, the hole could be `Dry' with probability 0.5, `Wet' with probability 0.3, and `Soaking' with probability 0.2. His monetary payoffs are given in the following table.

Table 3.1: Monetary Payoffs of Oil Wildcatter's Problem
  Drill Not Drill

The wildcatter also learned from the reports that the cost of drilling could be $150,000 with probability 0.2, $300,000 with probability 0.6, and $500,000 with probability 0.2. He can gain further relevant information about the underlying geological structure of this site by conducting seismic soundings. A cost control procedure that can make the probabilities of the `High' cost outcomes smaller (and hence, the probabilities of the `Low' cost outcomes larger) is also available. However, such information and control are quite costly, about $60,000 and $120,000, respectively. The wildcatter must also decide whether or not to take the sounding test or the cost control program before he makes his final decision: to drill or not to drill.

The oil wildcatter feels that he should structure and analyze his basic problem first: whether or not to drill. He builds a model that contains one decision stage named `Drill' (with two outcomes, `Drill' and `Not_Drill') and two chance stages named `Cost' and `Oil_Deposit'. A representation of the model is saved in three SAS data sets. In particular, the STAGEIN= data set can be saved as follows:

      /* -- create the STAGEIN= data set            -- */
   data Dtoils1;
      input _STNAME_ $12. _STTYPE_  $4. _OUTCOM_ $12.
            _SUCCES_ $12.;
   Drill       D   Drill       Cost
   .           .   Not_Drill   .
   Cost        C   Low         Oil_Deposit
   .           .   Fair        Oil_Deposit
   .           .   High        Oil_Deposit
   Oil_Deposit C   Dry         .
   .           .   Wet         .
   .           .   Soaking     .

The structure of the decision problem is given in the Dtoils1 data set. As you apply this data set, you should be aware of the following points:

The following PROBIN= data set contains the probabilities of events:

      /* -- create the PROBIN= data set               -- */
   data Dtoilp1;
      input _EVENT1 $12. _PROB1
            _EVENT2 $12. _PROB2
            _EVENT3 $12. _PROB3;    
   Low         0.2     Fair        0.6     High        0.2
   Dry         0.5     Wet         0.3     Soaking     0.2

Notice that the sum of the probabilities of the events `Low', `Fair', and `High' is 1.0. Similarly, the sum of the probabilities of the events `Dry', `Wet', and `Soaking' is 1.0.

Finally, the following statements produce the PAYOFFS= data set that lists all possible scenarios and their associated payoffs.

      /* -- create PAYOFFS= data set                  -- */
   data Dtoilu1;
      input (_STATE1-_STATE3) ($12.) ;  
      format _VALUE_ dollar12.0;

         /* determine the cost for this scenario */
      if _STATE1='Low' then _COST_=150000;
      else if _STATE1='Fair' then _COST_=300000;
      else _COST_=500000;

         /* determine the oil deposit and the          */
         /* corresponding net payoff for this scenario */
      if _STATE2='Dry' then _PAYOFF_=0;
      else if _STATE2='Wet' then _PAYOFF_=700000;
      else _PAYOFF_=1200000;

         /* calculate the net return for this scenario */
      if _STATE3='Not_Drill' then _VALUE_=0;
      else _VALUE_=_PAYOFF_-_COST_;

         /* drop unneeded variables */
      drop _COST_ _PAYOFF_;

   Low         Dry         Not_Drill
   Low         Dry         Drill
   Low         Wet         Not_Drill
   Low         Wet         Drill
   Low         Soaking     Not_Drill
   Low         Soaking     Drill
   Fair        Dry         Not_Drill
   Fair        Dry         Drill
   Fair        Wet         Not_Drill
   Fair        Wet         Drill
   Fair        Soaking     Not_Drill
   Fair        Soaking     Drill
   High        Dry         Not_Drill
   High        Dry         Drill
   High        Wet         Not_Drill
   High        Wet         Drill
   High        Soaking     Not_Drill
   High        Soaking     Drill

This data set can be displayed, as shown in Figure 3.1, with the following PROC PRINT statements:

      /* -- print the payoff table               -- */
   title "Oil Wildcatter's Problem";
   title3 "The Payoffs";

   proc print data=Dtoilu1;

Oil Wildcatter's Problem
The Payoffs

1 Low Dry Not_Drill $0
2 Low Dry Drill $-150,000
3 Low Wet Not_Drill $0
4 Low Wet Drill $550,000
5 Low Soaking Not_Drill $0
6 Low Soaking Drill $1,050,000
7 Fair Dry Not_Drill $0
8 Fair Dry Drill $-300,000
9 Fair Wet Not_Drill $0
10 Fair Wet Drill $400,000
11 Fair Soaking Not_Drill $0
12 Fair Soaking Drill $900,000
13 High Dry Not_Drill $0
14 High Dry Drill $-500,000
15 High Wet Not_Drill $0
16 High Wet Drill $200,000
17 High Soaking Not_Drill $0
18 High Soaking Drill $700,000

Figure 3.1: Payoffs of the Oil Wildcatter's Problem

The $550,000 payoff associated with the scenario `Low', `Wet', and `Drill' is a net figure; it represents a return of $700,000 for a wet hole less the $150,000 cost for drilling. Similarly, the net return of the consequence associated with the scenario `High', `Soaking', and `Drill' is $700,000, which is interpreted as a return of $1,200,000 less the $500,000 `High' cost.

Now the wildcatter can invoke PROC DTREE to evaluate his model and to find the optimal decision using the following statements:

      /* -- PROC DTREE statements              -- */
   title "Oil Wildcatter's Problem";

   proc dtree stagein=Dtoils1

      evaluate / summary;

The following message, which notes the order of the stages, appears on the SAS log:

   NOTE: Present order of stages: 
         Drill(D), Cost(C), Oil_Deposit(C), _ENDST_(E).

Oil Wildcatter's Problem

The DTREE Procedure
Optimal Decision Summary

Order of Stages
Stage Type
Drill Decision
Cost Chance
Oil_Deposit Chance

Decision Parameters
Decision Criterion: Maximize Expected Value (MAXEV)
Optimal Decision Yields: $140,000

Optimal Decision Policy
Up to Stage Drill
Alternatives or
Cumulative Reward Evaluating Value
Drill   $140,000*
Not_Drill   $0

Figure 3.2: Optimal Decision Summary of the Oil Wildcatter's Problem

The SUMMARY option in the EVALUATE statement produces the optimal decision summary shown in Figure 3.2.

The summary shows that the best action, in the sense of maximizing the expected payoff, is to drill. The expected payoff for this optimal decision is $140,000, as shown on the summary.

Perhaps the best way to view the details of the results is to display the complete decision tree. The following statement draws the decision tree, as shown in Figure 3.3, in line-printer format:

         /* plot decision tree diagram in line-printer mode */
      treeplot/ lineprinter;

Oil Wildcatter's Problem

The DTREE Procedure
Line-printer Plot

                                                                          | p=0.5  EV=  $-150,000   
                                                     Low                  |   Wet                   
                                                 | p=0.2  EV=   $300,000  | p=0.3  EV=   $550,000   
                                                 |                        | Soaking                 
                                                 |                        -----------------------E  
                                                 |                          p=0.2  EV= $1,050,000   
                                                 |                            Dry                   
                                                 |                        -----------------------E  
                                                 |                        | p=0.5  EV=  $-300,000   
                           Drill                 |  Fair                  |   Wet                   
                        |        EV=   $140,000  | p=0.6  EV=   $150,000  | p=0.3  EV=   $400,000   
                        |                        |                        | Soaking                 
                        |                        |                        -----------------------E  
                        |                        |                          p=0.2  EV=   $900,000   
                        |                        |                            Dry                   
----------------------D-|                        |                        -----------------------E  
        EV=   $140,000  |                        |                        | p=0.5  EV=  $-500,000   
                        |                        |  High                  |   Wet                   
                        |                        -----------------------C-|----------------------E  
                        |                          p=0.2  EV=   $-50,000  | p=0.3  EV=   $200,000   
                        |                                                 | Soaking                 
                        |                                                 -----------------------E  
                        |                                                   p=0.2  EV=   $700,000   
                                 EV=         $0                                                     

Figure 3.3: The Decision Tree

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Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.