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Controlling Solver

The following options control how Solver works:

  • Max Time— The amount of time Solver takes is a function of the size and complexity of the model, the number of changing cells and constraint cells, and the other Solver options you've chosen. If you find that Solver runs out of time before finding a solution, increase the number in this text box.


    Integer programming (in which you have integer constraints) can take a long time because of the complexity involved in finding solutions that satisfy exact integer constraints. If you find your models taking an abnormally long time to solve, increase the value in the Tolerance box to get an approximate solution. (See the Tolerance item later in this list.)

  • Iterations— This box controls the number of iterations Solver tries before giving up on a problem. Increasing this number gives Solver more of a chance to solve the problem, but it takes correspondingly longer.

  • Precision— This number determines how close a constraint cell must be to the constraint value you entered before Solver declares the constraint satisfied. The higher the precision is (that is, the lower the number is), the more accurate the solution is, but the longer it takes Solver to find it.

  • Tolerance— If you have integer constraints, this box determines what percentage of the integer Solver has to be within before declaring the constraint satisfied. For example, if the integer tolerance is set to 0.05%, Solver will declare a cell with the value 99.95 to be close enough to 100 to declare it an integer.

  • Assume Linear Model— In the simplest possible terms, a linear model is one in which the variables are not raised to any powers and none of the so-called transcendent functions—such as SIN() and COS()—is used. A linear model is so named because it can be charted as straight lines. If your formulas are linear, be sure to activate this check box because this will greatly speed up the solution process.

  • Assume Non-Negative— Activate this check box to force Solver to assume that the cells listed in the By Changing Cells list must have values greater than or equal to 0. This is the same as adding >=0 constraints for each of those cells, so it operates as a kind of implicit constraint on them. This is handy in models that use quite a few changing cells, none of which should have negative values.

  • Use Automatic Scaling— Activate this check box if your model has changing cells that are significantly different in magnitude. For example, you might have a changing cell that controls customer discount (a number between 0 and 1) and sales (a number that might be in the millions).

  • Show Iteration Results— Activate this option to have Solver pause and show you its trial solutions, as shown in Figure 16.8. To resume, click Continue from the Show Trial Solution dialog box.

    Figure 16.8. When the Show Iteration Results option is activated, Solver displays the Show Trial Solution dialog box so that you can view each intermediate solution.



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