Your LimeSurvey version: Version 5.6.13
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Hello, 

I have an equation that chooses a random number between 1 and 10 for each respondent. Unfortunately, the distribution of the number is not at all random. 80% of the respondents were assigned to numbers 1 to 4, 20% for each number.

Any recommendations?

Please find attached the lsg file.  

Please always provide .lss files, not questions or groups - they take too much time for the volunteers here to load up.

How big was your sample size?

I cannot reproduce the problem - here is what I see after a quick test of ~100 responses: