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Influence Diagrams, Decision Trees and Probability. Reference: Clemen & Reilly. Making Hard Decisions , 2nd ed. Chapter 3. Duxbury, 2001. NOTE: Some materials for this presentation courtesy of Dr. Dan Maxwell. Influence Diagrams. Another way to structure decision problems
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Influence Diagrams, Decision Trees and Probability Reference: Clemen & Reilly. Making Hard Decisions, 2nd ed. Chapter 3. Duxbury, 2001 NOTE: Some materials for this presentation courtesy of Dr. Dan Maxwell
Influence Diagrams • Another way to structure decision problems • Graphical representation • Considers: • Decisions to make • Uncertain events • Value of outcomes } Nodes in network Chance Node Deterministic Node Decision Node Value Node – Relations between nodes
Influence Diagrams Chance Relevance Value Decision Probabilistic Relevance Information Arc Deterministic Chance Functional Relevance
Representing Influence with Arrows F A B E The outcome of event A is relevant for assessing the chances associated with event B. The decision maker has information on the outcome of event E when making decision F. C D G H Decision C is relevant for assessing the chances associated with event D. Information from Decision G is used to make decision H.
Basic Risky Decision Outcomes: Market Up Market Down Market Activity Investment choice Payoff Market ChoiceOutcomePayoff Choices: Stocks Savings 200 200 500 -400 Up Down Up Down Savings Stocks
Evacuation Decision Possible Forecasts Will Hit Miami Won’t Hit Miami Outcomes: Hits Miami Misses Miami Hurricane Path Forecast Evacuation Decision Payoff ChoiceOutcomePayoff Choices: Evacuate Stay Safety High Cost Danger Low cost Safety Low Cost Hit Miss Hit Miss } Evacuate Stay Note: Decision made after forecast – Forecast is imperfect – Is forecast correct?
Strategy for Building Influence Diagrams • • No Recipe • • Start with very simple diagram • – Iteratively add detail • – Stop when enough to capture essence of problem • – There is art involved • • Common Mistakes • – Interpret as flowchart • - No sequential nature • - No cycles • – Influence not causation
Deterministic Nodes • • Additional nodes to aggregate intermediate results • • Emphasizes and simplifies structure of the ID ex. New Product Introduction (1) • Very simple representation • Value is the profit – derived from revenue and cost • Might not capture all relevant aspects of problem Revenue Cost Introduce Product Profit
Deterministic Nodes ex. New Product Introduction Units Sold (2) Fixed Cost Variable Cost Price Profit Introduce Product • More complex – captures more detail • Harder to understand and evaluate
Deterministic Nodes ex. New Product Introduction Units Sold (3) Fixed Cost Variable Cost Price Introduce Product Cost Revenue • Uses Deterministic Nodes • Sometimes denoted with double circle or as value node • Easier to understand and evaluate Profit
Deterministic Nodes ex. New Product Introduction Units Sold (3) Fixed Cost Variable Cost Price Introduce Product Cost Revenue • Uses Deterministic Nodes • Sometimes denoted with double circle or as value node • Easier to understand and evaluate Profit
Multiple Objectives • • Value of outcome depends on tradeoffs of competing objectives • • Can be represented in Influence Diagrams as follows: ex. FAA decision on Bomb Detection System Bomb Detection System Choice Detection Effectiveness Time to Implement Passenger Acceptance Cost • Build Additive Value Function Overall Satisfaction
Multiple Objectives • • Value of outcome depends on tradeoffs of competing objectives • • Can be represented in Influence Diagrams as follows: ex. FAA decision on Bomb Detection System Bomb Detection System Choice Detection Effectiveness Time to Implement Passenger Acceptance Cost Overall Satisfaction Measure
Multiple Objectives • • Value of outcome depends on tradeoffs of competing objectives • • Can be represented in Influence Diagrams as follows: ex. FAA decision on Bomb Detection System Bomb Detection System Choice Detection Effectiveness Time to Implement Passenger Acceptance Values Cost Overall Satisfaction Measure
Sequential Decisions • • Simplest is 2 decision sequence • • No cycles allowed in Influence Diagrams • • Sequential decisions are “strung together” ex. Orchard owner decision to protect trees from bad weather Weather Day 1 Weather Day 2 Weather Day n Forecast Day 1 Forecast Day 2 Forecast Day n Protect? Day 1 Protect? Day 2 Protect? Day n Payoff Day 1 Payoff Day n Payoff Day 1 Total Payoff
Sequential Decisions • • Simplest is 2 decision sequence • • No cycles allowed in Influence Diagrams • • Sequential decisions are “strung together” ex. Orchard owner decision to protect trees from bad weather Weather Day 1 Weather Day 2 Weather Day n Forecast Day 1 Forecast Day 2 Forecast Day n Protect? Day 1 Protect? Day 2 Protect? Day n Payoff Day 1 Payoff Day n Payoff Day 1 Total Payoff
Strategy for Building Influence Diagrams • • No Recipe • • Start with very simple diagram • – Iteratively add detail • – Stop when enough to capture essence of problem • – There is art involved • • Common Mistakes • – Interpret as flowchart • - No sequential nature • - No cycles • – Influence not causation
Solving Influence Diagrams • Step 1: Clean up Influence Diagram • No “Barren Nodes” • Only one Value Node (or one Super Value Node into which all the other value node feed) • No cycles • Step 2: Look for any Chance Nodes that: • Directly precedes the Value Node (only node) • Do not directly precede any other type node • Reduce these nodes by taking expected values • Value node inherits their predecessors
Algorithm (continued) • Step 3: Look for Decision node that: • Directly precedes the Value Node, and • Has as predecessors all other direct predecessors of the Value Node • If none, go to Step 5. • Else, reduce the node by choosing the optimum [expected] value. • Step 4: Go to Step 2 and continue until the Influence Diagram is completely solved • Step 5: You are here because you couldn’t reduce any chance nodes. • Reverse the arc between 2 chance nodes using Bayes Theorem and go to Step 2.
Solving Influence Diagrams • Influence Diagrams are usually solved using software • Freeware and Commercial software exists • In this class, we will use the Decision Tools software provided with the text • WARNING! Register software < 30 days • For exam, I won’t ask you to solve directly from a influence diagram