I want to thank all of you for your help the past few days. I now have all data sets imported, datetime columns added, and distribution stats calculated for each. No errors. My searches on the web for what to do when problems() produces no results, and the few comments on my stackexchange post, were not helpful. So I fell back on how I debugged FORTRAN code in the 1970s: cut the data file in half and run that. If the issue still exists, cut that half in half again. Rinse and repeat until the problem line is found. It turned out that one 416K line data file had a few double commas between columns about line 1530 in the file. Finding those double commas showed me the problem and the emacs search-and-replace changed the two commas to a single comma. Now all's well. It took another mug of coffee and a lunch break to work my way down to the beginning of the large file to the errors, but it worked just as it did before FORTRAN IV had a debugger. :-) Stay well, all! Rich