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Message: <blockquote data-quote="CapnZapp" data-source="post: 6987277" data-attributes="member: 12731">The probability of one monster with +5 passing a DC 15 check is indeed 55%.However, the probability of two monsters both passing is only 55% x 55% or 30,25%. More generally, the probability of N monsters all passing is 55%^N. For N=5 the probability is only about 5%. If you only make the check when the monsters are in position, thus turning the check into the answer to the question "who's surprised?" this might be okay. But if you use this check to determine if the heroes spot the monsters already when they try to sneak up on the party, when there is still time to do something about the ambush (such as fleeing etc), this doesn't work at all, since it means only one in twenty ambushes actually happens.---If I agree that your number 55% is good enough for me too, I want the math to result in 55% of all ambush attempts to actually succeed. What happens on the "surprise round" is an entirely secondary concern to me.Now, using group checks would get us there. The probability of five out of five checks succeeding might only be 5%, but the probability of at least three out of five checks succeeding is 59%.---Now the statistics of group checks swings rather heavility based on the number in the group. I found an old thread complaining about how the probabilities actually decrease if a group of two unskilled characters are joined by a third, even if that character is highly skilled. The reason, of course, is that the addition of the third character means that now 66% of the group needs to succeed compared to only 50% of the original two-man group. That is to say, one out of the two unskilled characters STILL needs to succeed: all the addition of the expert has done is added the risk of that expert failing (however small). To illustrate this, compare the above scenario to when one of the five monsters decide to hang back. The probability of setting up the ambush drops from 59% to 39%!Probability of 3 to 5 checks out of 5 making it: 59%Probability of 3 to 4 checks out of 4 making it: 39%So while group checks should probably be standard, they do add a measure of unpredictability that might catch DMs unawares.</blockquote>

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