Ideally, you would collect the input into a database or a csv file, and do the analysis offline in a statistics package like SPSS.

Most databases can do rudimentary statistical analysis, so if that's enough for your purpose you might as well use it. Some SQL keywords are avg, stddev, group by, rollup, cube.

Alternatively, you might be interested in PDL, the Perl Data Language.

XML is definitely not the best storage mechanism for this.