On allstat a few years ago, a weird example was mentioned. Enter a column of zeros and set a cell to contain their sum. Now change one cell to O (upper case O). Now change another cell to l (lower case l). What happens? What would be reasonable behaviour */for statistical purposes/*? Do you want to trust this software?I tried this in excel and wasn't really sure what the problem was - it was just the sum. He doesn't really explain the "reasonable behavior" he is looking for? It made sense to me ?? (And that worries me!) FrankHarrell: I haven't tried that myself. I take it that the non-numerics were not properly excluded from the sum or more likely that a sum of non-missing values was computed instead of giving an answer of NA that would alert the analyst about bad data in the column. There may also be problem when counting the number of non-missing entries for the purposes of getting a mean.