Sorry, I meant PearsoChiSquaredTest (I edited it now). My question was really whether there is a command in Mathematica to do this or not (so I did not intend it to be a theoretical question). This seems to be a natural question, so I was wondering if I missed a command or not.

This is indeed unfortunate. This makes it pointless for me to use the input box this way. I need to rework everything to input coefficients instead of the full function. It would have been nice to be able to visualize more general functions, though.

I would like to ask one more question. I saved the output as a CDF file (I would like to show it to people without access to Mathematica), but in the CDF file the content of the input box does not change the graph. I always see the the grapf of z^2 no matter I write in the input box. Is this a bug in the CDF player (I use version 10) or do I need to change something in the code?

Thank you very much, this is very helpful. In my original code, the z-replacement does not work when I first run the code, but it does work if I redefine the function in the input box. I could not figure out the reason for this, but since this was not a major issue, I did not want to ask two questions at the same time. Your solution fixed this problem, too, thanks.

Sorry, I am not sure how to attach a screenshot, so I try to explain what is happening. On my system (Linux Mint 17.2, Mathematica 10) when I type t^2, it immediately moves the 2 in the superscript. Probably it is an operating system issue, but still, it would be good to know if there is an option to the input field to override this. Also, t^2 (with the 2 in the superscript) is not recognized by Mathematica as an expression, rather it treats it as a variable for some reason.