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Chapter 7. What Sample Sizes Do We Need? > Additional Applications of the Model

Additional Applications of the Model

There are other interesting, although somewhat controversial, things you can do with this model.

Estimating the Composite Value of p for Multiple Problems or Other Events

An alternative to selecting the lowest value of p for events that you want to have a good chance of discovering is to estimate a composite value of p, averaged across observed problems and study participants. For example, consider the hypothetical results shown in Table 7.3.

Table 7.3. Hypothetical Results for a Formative Usability Study

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Note: x = specified participant experienced specified problem.

For this hypothetical usability study, the composite estimate of p is 0.5. There are a number of ways to compute the composite, all of which arrive at the same value. You can take the average of the proportions, either by problems or by participants, or you can divide the total number of cells in the table (100 − 10 participants by 10 discovered problems) by the number of cells filled with an “x” (50).


  

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