Buy or build" is simply the decision. Project A: Outside building. The cost of the decision if you "buy" the work outside of your company is $175,000. Based on your probability of completion by a given date, you apply the 85 percent certainty to the "strong" finish for the Buy branch of the tree. Because your 85 percent certain, you're also 15 percent uncertain; this value is assigned to the "weak" value on the Buy branch