Frequently Asked Questions about Excel Solver
Below is a partial list of some of the issues that might come up when you are using Solver. If you find others that you think people should know about please email email@example.com.
- Where can I get some instructions on how to use Solver?
- There is an article in the readings book (Reading #1, page 11) which gives a complete worked-through example.
- How do I get Solver working?
- No. The only reason you may not see "Solver" in your Tools menu is that Solver was not loaded when you originally installed Excel. In that case, go to Tools|AddIns and check off "Solver Add In". Then click OK and return to the Tools menu. Now Solver should appear.
- When do I check off next to "Assume Linear Model" ?
- Whenever the model you are solving is a linear model. A model is linear if decision variables are only multiplied by fixed numbers (constants), variables are not multiplied together, there are no squares, logs, if statements, powers etc. used directly in the model (you are allowed to use non-linear formulas to calculate parts of your spreadsheet, but they cannot be part of the constraints or decision variables as specified in Solver).
- What does it mean when solver says "The conditions for Assume Linear Model are not satisfied"?
- This means that you have not written a linear model. Somewhere you have either multiplied decision variables, or used non-linear expressions (powers, logs, 1/x). If the model is supposed to be linear, then you need to fix your model. If the model is supposed to be nonlinear, then you need to uncheck "Assume Linear Model" in the Solver Options.
- What does it mean when solver says "The Set Cells do not converge"?
- This means that your problem has an unbounded or infinite solution. That means that you probably modelled it wrong, and Excel is able to increase profits to infinity, or decrease cost to negative infinity, or something like that. To debug this: make sure your constraints are correctly entered.
- What does it mean when I check off "Assume Non-Negative" in the Options?
- When you do that you are telling the Solver to restrict all the decision variables (changing cells) to be non-negative (=positive or 0). In many decision models, negative decision variables do not make sense and this is the easiest way to enforce that restriction. (Another way would be to put in constraints that the decision variables be >=0).
- How can I specify several ranges in the "changing cells"?
- You can do this by separating them with a comma: e.g. A4:B4,C9:D11.