If You Aren't Getting the Solution You Expect

This topic is intended to help you diagnose problems with your Solver models. The most important step you can take to deal with potential Solver problems is to start out with a clear idea of the type of optimization model you are creating, how it relates to well-known problem types, and whether yours is a linear, nonlinear or integer programming problem -- as discussed in previous topics. If you then build your model in a well-structured, readable and efficient form -- as outlined elsewhere in this Helpfile -- diagnosing problems should be relatively easy. But at times you may be "surprised" by the results you get from the Solver.

If the Solver stops with a solution (set of values for the decision variables or changing cells) which is different from what you expect, or what you believe is correct, follow the suggestions below. You can usually narrow down the problem to one of a few possibilities.

Check the Solver Completion Message shown in the Solver Results dialog. Users sometimes contact Frontline Systems about "wrong solutions", but they don't know which Solver Completion Message they received. This is crucial to diagnosing the problem.

Consider carefully the possibility that the solution found by the Solver is correct, and that your expectation is wrong. This may mean that what your model actually says is different from what you intended.

Check the "Show Iteration Results" box in the Solver Options dialog and re-solve. The Solver will pause with the message "Solver paused, current solution values displayed on worksheet." Click Continue to see the path towards the solution taken by the Solver.

Consider the possibilities for poorly scaled models, role of the Tolerance option for integer problems, limitations on nonlinear problems, and problems with discontinuous functions outlined below.