2.2 Incorporate uncertainty (outcome probability)

You can now incorporate the relative outcome probability, or uncertainty, associated with each chance event. You can express probabilities as percentages or as decimal fractions. This primer adopts the common decision tree convention of using decimal fractions from (0.0) to (1.0), in which (1.0) = 100%.

In the Really Big Ideas scenario, the smoke and fire detector has a 50% chance of success, and therefore a 50% chance of failure. Therefore, write (0.5) on the success branch and (0.5) on the failure branch. The motion detector has an 80%, or (0.8) chance of success, and therefore a 20%, or (0.2) chance of failure. Write these values on their respective branch lines, as in figure 2.2.

Fig 2.2 Write the probability for each outcome branch. You can express a probability as a decimal fraction in parentheses.


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