IDEA happens to be one way. The IDEA toolkit from WonderWare is a set of C Programming interfaces, which you can use to write your own drivers/programs that can access the WonderWare tag database.
Probably not the way you want to go.
The simplest way to accomplish getting data for Crystal might be to load the Microsoft Jet (Access Database) engine on the computer, and use the InTouch SQL Access functions to write to that based on data change scripts, condition scripts, etc. SQL Access can handle reading/writing/creating from within InTouch.
Another way, if you are just looking for historical data, is to have the HistData program from WonderWare (included with InTouch) periodally write out a .CSV file of logged historical data, then import that to Crystal.
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SPC stands for 'Statistical Process Control', and can be used for exactly that... Monitoring a process, determining what the operating conditions and limits are, and then modifying the process parameters to bring it into the desired Upper/Lower control limit band. SQC (Statistical Quality Control) is essentially the same thing, but without 'closing the loop' and modifying the process.
SPC is NOT for the beginner, or faint-hearted. Developing a functional SPC system requires a great deal of knowledge about the process, and is something I normally work with process engineers on over a period of time. I hate it so much, as a matter of fact, that I usually just point Process Engineers to other SQC Packages, then publish historical data (again, in a CSV file) that they can import and play with.