An entry in the simulation preferences window: label says "Threshold", then there's a spinner from 0 to 100, which is fed into a property along the lines of "EFFECT_ODDS_THRESHOLD". This is then used in the simulation to determine if/when an effect is even allowed to roll for a chance to occur. Any likelihood that is below the threshold is told "no, you didn't happen" regardless of the Math.random() result. This will give the user a way to give bias against unlikely events.
See here for the reddit thread
Basically:
An entry in the simulation preferences window: label says "Threshold", then there's a spinner from 0 to 100, which is fed into a property along the lines of "EFFECT_ODDS_THRESHOLD". This is then used in the simulation to determine if/when an effect is even allowed to roll for a chance to occur. Any likelihood that is below the threshold is told "no, you didn't happen" regardless of the Math.random() result. This will give the user a way to give bias against unlikely events.