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Your LimeSurvey version: 5.3.17
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Survey theme/template: Inherit [fruity]
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Read through manual and still confused.
1) How can I set a conditional response on a multiple choice question? 
For example, Q1 has two possible 'yes' answers and I'd like a positive response to either 'yes' to lead to the same 1-2 additional question(s).  (Since Lime doesn't use JumpBranching, do I also need to set a condition that a 'no' response would then skip the additional questions? How?)
2) How can I run the survey (without saving a response) to see if the conditions I've set are working? Preview survey doesn't (unless I've botched setting the conditional question.)

1) Relevance equation:
2) Yes, conditions/relevance do work in non-activated surveys.

To answer your second question: Of course it does.

And the first:
Always remember:
In "condition" is entered a logical term that is either TRUE or FALSE.
If TRUE, the group/question/subquestion is displayed, else not.

Now there is the usual confusion about "multiple choice question".
Is it a single punch question "list(radio)", etc. or a "multiple" question?

1. Single question
let's say the first and the third answer option make the next question appear.
So if the first option is selected or the third option is selected, your condition has to be TRUE.
Obviously the condition is : Q1==1 OR Q1= (assuming you used numerical codes)

2. Multiple question
I think you know already that in a multiple question the result of each subquestion is saved as: "Y"=selected, ""=not selected.
Now the same condition: The first and/or the third subquestion is selected -> next question displayed.
Q1_SQ001=="Y" OR Q1_SQ003=="Y"


And here the result
 

 



Joffm