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Hi, is it possible to create a mandatory option based on relevance equation? Please let me know, thank you.

As far as I know: no. Not at this moment. But it would be a good idea.
0 for not mandatory
everything between 0 and 1 for soft mandatory
1 for mandatory?

Use a question validation equation.

Thank you for the suggestion, can you give an example of how to use a question validation equation?

Read the manual about question types-
Each one contains a chapter about question validation.
[url] www.limesurvey.org/manual/Question_type_...28em_validation_q.29 [/url]

Then there are examples here
[url] www.limesurvey.org/manual/ExpressionScri...alidation_q_in_array [/url]
[url] www.limesurvey.org/manual/ExpressionScri...Checkbox.22_question [/url]

And here you find a demo survey
[url] www.limesurvey.org/manual/ExpressionScri...n_Types_Demonstrated [/url]
Either you download and import  it or you may study it in the demo installation of LimeSurvey
[url] demo.limesurvey.org/index.php?r=admin/authentication/sa/login [/url]

In short:
In the question validation you enter a logical term that is either TRUE or FALSE.
If TRUE the question is answered correctly, if FALSE it is not and an error message is displayed.
Example:
Your question validation is: Q1==1
If the respondent entered "1" in Q1 the equation is TRUE, otherwise FALSE.

It is like relevance equations where a question/group is only displayed if the equation is TRUE.

In your case where you want to switch the mandatory option you obviously will check that the question is not empty.

Joffm