What tools are there for analysing experimental data with a number of variables to attempt to optimise parameters for a particular parameter? This question is purposely general - I'm more asking for tools that I should look at in the future, than tools to use now.
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As far as free tools go, I second Dirk's suggestion of R, but I'd like to mention Python and the SciPy library (in particular, the scipy.optimize package) as a possible alternative. It would help of you could be more specific as to what kind of data analysis you are trying to do so that we can be more specific with our recommendations.