I think, perhaps, I may have found a bug in the Constant Sum question. I set up a question with 18 options. The min value for each was 0 and the max value for each was 100. The total sum was set at 100. The question was set as required.
Now, when you first got to the page and entered your values, it behaved exactly as it should (alerting you if the total was above/below the expected). However, if you leave the survey or navigate to a different page and then come back to the page, everything goes weird. The values will pre-fill from the database as expected. If you click on the Next button, something happens to where the validation no longer passes. You will get fairly random addition. For example, I entered 95 in one box and 5 in another box. The error message said that they currently add up to 270. I changed the 95 to a 90 and entered a 1 in an additional 5 boxes and it then told me that they currently add up to 180.
I have dug through the code and tried to echo out to see what was going on, but I couldn't figure it out. This is on iJoomla Survey 3.0.9.