Chapter 5. Calibrated Estimates: How Much Do You Know Now?

The most important questions of life are indeed, for the most part, really only problems of probability.

Pierre Simon Laplace, Théorie Analytique des Probabilités, 1812

How many hours per week do employees spend addressing customer complaints? How much would sales increase with a new advertising campaign? Even if you don't know the exact values to questions like these, you still know something. You know that some values would be impossible or at least highly unlikely. Knowing what you know now about something actually has an important and often surprising impact on how you should measure it or even whether you should measure it. What we need is a way to express how much we know now, however little that may be. To do that, we need a way to know if we are any good at expressing uncertainty.

One method to express our uncertainty about a number is to think of it as a range of probable values. In statistics, a range that has a particular chance of containing the correct answer is called a "confidence interval" (CI). A 90% CI is a range that has a 90% chance of containing the correct answer. For example, you can't know for certain exactly how many of your current prospects will turn into customers in the next quarter, but you think that probably no less than three prospects and probably no more than seven prospects will sign contracts. If you are 90% sure the actual number will fall between three and seven, then we can say you have ...

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