Why does the calibration function in NumXL shows the constraint's rhs of 0.9999?

When you click on the "Calibration" icon in the toolbar (or menu), NumXL prepare all inputs of the solver using the selected model, and add one constraint  (As shown below):

MS Excel Solver showing constraints with 0.999 limit value

The constraint is added to make sure the solver search only for a stable model (e.g. invertible, characteristics root outside unit-circle, etc.). NumXL has functions for the different models that checks the stability of the model and returns either 0/1.

Ideally, the constrain above should state something like ($J$43=1), but due to an issue in the Excel Solver using VBA, we introduced a work-around to overcome situations where the first constraint is missing or dropped.

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