Excel Data Analysis For Dummies. Paul McFedries
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FIGURE 2-13: The Solver Results dialog box and the solution to the break-even problem.
Adding constraints to Solver
The real world puts restrictions and conditions on formulas. A factory might have a maximum capacity of 10,000 units a day, the number of employees in a company can’t be a negative number, and your advertising costs might be restricted to 10 percent of total expenses.
Similarly, suppose that you’re running a break-even analysis on two products, as I discuss in the preceding section. If you run the optimization without any restrictions, Solver might reach a total profit of 0 by setting one product at a slight loss and the other at a slight profit, where the loss and profit cancel each other out. In fact, if you take a close look at Figure 2-13, this is exactly what Solver did. To get a true break-even solution, you might prefer to see both product profit values as 0.
Such restrictions and conditions are examples of what Solver calls constraints. Adding constraints tells Solver to find a solution so that these conditions are not violated.
Here’s how to run Solver with constraints added to the optimization:
1 Choose Data ⇒ Solver.Excel opens the Solver Parameters dialog box.
2 Use the Set Objective box, the To group, and the By Changing Variable Cells box to set up Solver as I describe in the preceding section, “Optimizing a result with Solver.”
3 Click Add.Excel displays the Add Constraint dialog box.
4 In the Cell Reference box, enter the address of the cell you want to constrain.You can type the address or select the cell on the worksheet.
5 In the drop-down list, select the operator you want to use.Most of the time, you use a comparison operator, such as equal to (=) or greater than (>). Use the int (integer) operator when you need a constraint, such as total employees, to be an integer value instead of a real number (that is, a number with a decimal component; you can't have 10.5 employees!). Use the bin (binary) operator when you have a constraint that must be either TRUE or FALSE (or 1 or 0).
6 If you chose a comparison operator in Step 5, in the Constraint box, enter the value by which you want to restrict the cell.Figure 2-14 shows an example of a completed Add Constraint dialog box. In the example model, this constraint tells Solver to find a solution such that the product profit of the Inflatable Dartboard (cell B12) is equal to 0.FIGURE 2-14: The completed Add Constraint dialog box.
7 To specify more constraints, click Add and repeat Steps 4 through 6, as needed.For the example, you add a constraint that asks for the Dog Polisher product profit (cell C12) to be 0.
8 Click OK.Excel returns to the Solver Parameters dialog box and displays your constraints in the Subject to the Constraints list box.
9 Click Solve.
10 In any Show Trial Solution dialog box that appears, click Continue to move things along.Figure 2-15 shows the example break-even solution with the constraints added. Note that not only is the Total Profit cell (B14) set to 0, but so are the two Product Profit cells (B12 And C12).
11 Select the Keep Solver Solution option.If you don’t want to accept the result, select the Restore Original Values option instead.
12 Click OK.
FIGURE 2-15: The Solver Results dialog box and the final solution to the break-even problem.
Save a Solver solution as a scenario
Whenever you have a spreadsheet model that uses a coherent set of input values — known as changing cells — you have what Excel calls a scenario. With Solver, these changing cells are its variable cells, so a Solver solution amounts to a kind of scenario. However, Solver does not give you an easy way to save and rerun a particular solution. To work around this problem, you can save a solution as a scenario that you can then later recall using Excel’s scenario manager feature.
Follow these steps to save a Solver solution as a scenario:
1 Choose Data ⇒ Solver.Excel opens the Solver Parameters dialog box.
2 Use the Set Objective box, the To group, the By Changing Variable Cells box, and the Subject to the Constraints list to set up Solver as I describe in the “Optimizing a result with Solver” section, previously in this chapter.
3 Click Solve.
4 Anytime the Show Trial Solution dialog box appears, choose Continue.When the optimization is complete, Excel displays the Solver Results dialog box.
5 Click Save Scenario.Excel displays the Save Scenario dialog box.
6 In the Scenario Name dialog box, type a name for the scenario and then click OK.Excel returns you to the Solver Results dialog box.
7 Select the Keep Solver Solution option.If you don’t want to accept the result, select the Restore Original Values option instead.
8 Click OK.
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