I want to be validate the answer based on the answer of previous question.
For example:
Q1: How old are you?
Q2: How old are your son?

Now, the answer of Q2 should be lower than the answer of Q1.
How can I do this validation?

I got it.
You can check this tutorial page: docs.limesurvey.org/Expression+Manager+Sample+Surveys
If you are not patient, check this survey: docs.limesurvey.org/tiki-download_wiki_a...attId=115&download=y

Q2 < Q1 in relevance equation.

docs.limesurvey.org/Expression+Manager

Thank you for your answer, but as I know, the relevance equation helps to show or hide the question but not to validate the answer.

OUps .... sorry.

Question validation equation .... in advanced settings of the question

Sorry, but the validation equation is a regular expression. Can we use a SGQA identifier in the regular expression?

luayessa wrote: To test, in a numeric question type, if Q1 < Q2, use it directly in advanced setting/Question validation equation

You can use regular expression to with EM in "Question validation equation" : RegExp(blabla)

Not validation equation but "Question validation equation"
See docs.limesurvey.org/Expression+Manager#Validation and docs.limesurvey.org/Advanced+question+settings#em_validation_q