Subjective Probability

Subjective Probability

What about statements like "there is a 20% chance of rain or snow today in the higher elevations"? A statement of this type does not express a relative frequency probability. We do not have an idea of the population (number of times it rains or snows in a time period), so there is no way to see the proportionality of the outcome to the total population. Statements of this type, quite commonly made, express a subjective notion of probability. We will see that statements of this type express a "confidence." We arrive at "confidence" by accumulating probabilities over a range. We discuss more about "confidence" later.

^[2]In the real world, events "5" and "7" could be the names given to two events in a "tree" or hierarchy of events requiring study. For example, events "5" and "7" could be error conditions that might occur. Rolling the dice is equivalent to simulating operational performance. Dr. David Hulett suggested the fault tree to the author.

^[3]A might be one approach in a project and B might be another approach. Such a situation comes up often in projects in the form of "if, then, else" conditional branching.

^[4]A finish-to-start precedence is a notation from precedence diagramming methodology. It denotes the situation that the finish of the first task is required before the start of the second (successor) task can ensue. It is often referred to as a "waterfall" relationship because, when drawn on paper, it gives the appearance of a cascade from one task to the other. More on critical path scheduling can be found in Chapter 6 of A Guide to the Project Management Body of Knowledge. ^[5]

^[4]A Guide to the Project Management Body of Knowledge (PMBOK® Guide) — 2000 Edition, Project Management Institute, Newtown Square, PA, chap. 6, p. 69

Subjective Probability

Subjective Probability

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