## More on Introducing Uncertainty

So far, we've modified our spreadsheet model to introduce **uncertainty** for Sales Volume at cell F5 and Selling Price at F6. Now we'll deal with Unit Cost. We have not just three, but many possible values for this variable: It can be anywhere from $5.50 to $7.50, with a most likely cost of $6.50. A crude but effective way to model this is to use a *triangular* distribution. **Risk Solver** provides a function called **PsiTriangular()** for this distribution.

With cell F7 (representing Unit Cost) selected, from the Risk Solver Ribbon we click the **Distributions** button and then select **Triangular** from the **Common** distributions gallery. (Unlike a discrete distribution, a sample drawn from a *continuous* distribution can be *any* numeric value, such as 5.8 or 6.01, in a range.)

Risk Solver displays the **Uncertain Variable** dialog with a chart of the **triangular** distribution -- initially with parameters: min 0, likely 1 and max 3. We want to edit these parameters -- but instead of entering fixed numbers, we'll use the range selector icon at the right of each field to **select cells** containing the parameters: min B9 ($5.50), likely B10 ($6.50) and max B11 ($7.50). This means that on each trial, we'll draw a number between $5.50 and $7.50, where $6.50 is the *most likely* value to be drawn -- as shown in the chart of the triangular distribution below. Hence $6.45 and $6.55 are more likely than $5.55 or $7.45 -- but *any* of these and other numbers has a chance of being drawn on each trial. When we click the **Save** icon in the dialog toolbar, a formula **=PsiTriangular(B9,B10,B11)** is written to F7. (Note that you could also type the formula **=PsiTriangular(B9,B10,B11)** in cell F7.) We now have a second **uncertain variable** in our model.

Next, we'll move on to define an **Uncertain Function**. But we can do much more than shown here with the Uncertain Variable dialog: The view above shows all of its panes open. You can **browse** different distributions, **shift** and **truncate** a distribution, see each distribution's Probability Density Function (**PDF**), Cumulative Density Function (**CDF**) or **Reverse CDF**, see **statistics** and **percentiles** for the distribution, automatically **fit a distribution** to user-supplied data, and **customize** the chart.

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