I am not an expert on this kind of statistics processing, but a search in IDL 6.1 Online Help for the term "chi-square" reveals that there are only a half-dozen or so functions (e.g. XSQ_TEST, CHISQ_CVF, CHISQ_PDF, etc.) , whose descriptions and/or syntax that you need to study to determine if old IDL has the functionality that you are describing. I am pretty sure that IDL 6.3 users with licensing for the new IDL Analyst math and statistics processing libraries have access to this functionality. Here is the description for the 'IMSL_SHISQCDF' function that is included in that new library: "The IMSL_CHISQCDF function evaluates the chi-squared distribution or non-central chi-squared distribution." James Jones