Assuming we want to add the validation such that the total of this question matches with the total of some other question: In this case, we can add math operation in custom validation

https://community.qualtrics.com/XMcommunity/discussion/comment/39208#Comment_39208Really thanks for your help. I am sorry and I am so confused about how to use the custom validation.
May I know where can find the user guide for the custom validation?
Many thanks.‍♂️

Qualtrics has not created a support page for this specific use case. Check out this support page on Math operations: https://www.qualtrics.com/support/survey-platform/survey-module/editing-questions/piped-text/math-operations/
Once you have your math operations set, you can then use it in Custom Validation in the Constant Sum question: https://www.qualtrics.com/support/survey-platform/survey-module/editing-questions/piped-text/math-operations/