Fitting to system of equations with 3 variables, but only two variable have data

Are you looking for an approximate solution for an over-determined system (fitting), or is there an exact solution to be found by root finding? Some more details about what you're trying to do would be helpful.

To me it sounds like you need curve-fitting to do this. Most likely you can look at File - Example Experiments - Curve Fitting - Batch Curve Fitting Demo since I think this is best suited for your problem... BUT that is a wild guess, there are lots of other options on how to fit in Igor that are simpler than this...

I think what tony said is a good approach, give us some more information and maybe we'll be able to help...

Best,

Johan

Oups: I ment Global Fit, not Batch Fitting... sorry about that.

If I understand your situation, you might be able to do an implicit fit (DisplayHelpTopic "Fitting Implicit Functions").

Thank you. This is likely to be helpful, but I still have the problem of an unmeasured variable that I don't want to fit ("a" can't be eliminated and I don't have data for it).  I've been a little slow to respond to the reasonable requests for more information because I've discovered more issues as I dig in deeper to the problem. I will hopefully write back soon with enough information to make the problem clear and concrete.

Let's see- if you have a model that allows you to compute P and G given a, if you have values of P and G, you could use a root finder to get a numerically. But I suppose there are model coefficients that you want to estimate from the data, so you don't actually have a complete model to feed into the root finder...

Sounds like an underdetermined system where you need to solve for both a and the coefficients, with some sort of minimum norm to constrain the a values.

Now I've said everything I'm even a little bit competent to say :)