Chapters 19 – 20: Dealing With Uncertainties TPS Example: Uncertain outcome Decision rules for complete uncertainty Utility functions Expected value of perfect information Statistical decision theory The value-of-information procedure

TPS Decision Problem 7 OS Development Options: Option B-Conservative (BC) –Sure to work (cost=$600K) –Performance = 4000 tr/sec Option B-Bold (BB) –If successful,Cost=$600K Perf.= 4667 tr/sec –If not,Cost=$100K Perf.= 4000 tr/sec Which option should we choose?

Operating System Development Options Option BB (Bold) SuccessfulNot Successful Option BC (Conservative) Option A Performance (tr/sec) Value ($0.3K per tr/sec) Basic cost Total cost Net value NV NV relative to Option A

Payoff Matrix for OS Options BB, BC State of Nature AlternativeFavorableUnfavorable BB (Bold) BC (Conservative)50

Decision Rules for Complete Uncertainty Maximin Rule. Determine minimum payoff for each option. Choose option maximizing the minimum payoff. Maximax Rule. Determine max-payoff for each option. Choose option maximizing max-payoff. Laplace Rule. Assume all states of nature equally likely. Choose option with maximum expected value. BB = 0.5(250) + 0.5(-50) = 100 BC = 0.5(250) + 0.5(50) = 50

Difficulty with Laplace Rule FavorableU1U1 U2U2 Expected Value BB BC50 U 1 : performance failure U 2 : reliability failure

Breakeven Analysis Treat uncertainty as a parameter P= Prob (Nature is favorable) Compute expected values as f(P) EV (BB) = P(250) + (1-P)(-50) = P EV (BC) = P(50) + (1-P)(50) = 50 Determine breakeven point P such that EV (BB) = EV (BC): P = 50, P=0.333 If we feel that actually P>0.333, Choose BB P<0.333, Choose BC

Utility Functions Is a Guaranteed $50K The same as A 1/3 chance for $250K and A 2/3 chance for -$50K?

Utility Function for Two Groups of Managers Utility Payoff ($M) Group 1 Group 2

Possible TPS Utility Function (-80,-25) (-50,-20) (50,5) (100,10) (170,15) (250,20)

Utility Functions in S/W Engineering Managers prefer loss-aversion –EV-approach unrealistic with losses Managers’ U.F.’S linear for positive payoffs –Can use EV-approach then People’s U.F.’S aren’t –Identical –Easy to predict –Constant

TPS Decision Problem 7 Available decision rules inadequate Need better information Info. has economic value State of Nature AlternativeFavorableUnfavorable BB (Bold) BC (Conservative)50

Expected Value of Perfect Information (EVPI) Build a Prototype for $10K –If prototype succeeds, choose BB Payoff: $250K – 10K = $240K –If prototype fails, choose BC Payoff: $50K – 10K = $40K If equally likely, Ev = 0.5 ($240K) ($40K) = $140K Could invest up to $50K and do better than before –thus, EVPI = $50K

However, Prototype Will Give Imperfect Information That is, P(IB|SF) = 0.0 Investigation (Prototype) says choose bold state of nature: Bold will fail P(IB|SS) = 1.0 Investigation (prototype) says choose bold state of nature: bold will succeed

Suppose we assess the prototype’s imperfections as P(IB|SF) = 0.20,P(IB|SS) = 0.90 And Suppose the states of nature are equally likely P(SF) = 0.50P(SS) = 0.50 We would like to compute the expected value of using the prototype EV(IB,IC) = P(IB) (Payoff if use bold) +P(IC) (Payoff if use conservative) = P(IB) [ P(SS|IB) ($250K) + P(SE|IB) (-$50K) ] + P(IC) ($50K) But these aren’t the probabilities we know

How to get the probabilities we need P(IB) = P(IB|SS) P(SS) + P(IB|SF) P(SF) P(IC) = 1 – P(IB) P(SS|IB) = P(IB|SS) P(SS)(Bayes’ formula) P(IB) P(SF|IB) = 1 – P (SS|IB) P(SS|IB) = Prob (we will choose Bold in a state of nature where it will succeed) Prob (we will choose Bold)

Net Expected Value of Prototype PROTO COST, $K P (PS|SF)P (PS|SS)EV, $KNET EV, $K NET EV, $K PROTO COST, $K

Conditions for Successful Prototyping (or Other Info-Buying) 1.There exist alternatives whose payoffs vary greatly depending on some states of nature. 2.The critical states of nature have an appreciable probability of occurring. 3.The prototype has a high probability of accurately identifying the critical states of nature. 4.The required cost and schedule of the prototype does not overly curtail its net value. 5.There exist significant side benefits derived from building the prototype.

Pitfalls Associated by Success Conditions 1.Always build a prototype or simulation –May not satisfy conditions 3,4 2.Always build the software twice –May not satisfy conditions 1,2 3.Build the software purely top-down –May not satisfy conditions 1,2 4.Prove every piece of code correct –May not satisfy conditions 1,2,4 5.Nominal-case testing is sufficient –May need off-nominal testing to satisfy conditions 1,2,3

Statistical Decision Theory: Other S/W Engineering Applications How much should we invest in: –User information gathering –Make-or-buy information –Simulation –Testing –Program verification How much should our customers invest in: –MIS, query systems, traffic models, CAD, automatic test equipment, …

The Software Engineering Field Exists Because Processed Information Has Value