DiscussWorldIssues - Socio-Economic Religion and Political Uncensored Debate - View Single Post - math guys: how does one figure out a standard deviation

**exchpaypaleg** · 05-09-2007, 07:44 PM

Originally posted by self biased
what if i'm only looking for certain values? for example, what if i'm looking for a 3+ on a d6 on 15 dice? odds say that ten of those fifteen will be a three or better. dropping some dice now i get: 2, 3, 1, 1, 2, 2, 1, 4, 6, 2, 4, 6, 6, 6, and 1. more than half my values are less than my target of 3+.

is there any way of shortcutting this? more often than not, we're looking for 3+ or 4+ on anywhere from four to thirty-six dice. 15 choose 10 divided by 2^15 for 10 4+'s. You'll need a calculator.

In general, if you're making n rolls and you want to know the probability of getting k of some result (with a probability p of getting that result on any single roll, e.g. p=1/2 for a 4+), use the formula (n choose k) * p^k * (1-p)^k.

edit: (n choose k) = n! / ((n-k)! * k!), where ! = factorial.

05-09-2007, 07:44 PM	#28
exchpaypaleg Join Date Oct 2005 Posts 435 Senior Member	Originally posted by self biased what if i'm only looking for certain values? for example, what if i'm looking for a 3+ on a d6 on 15 dice? odds say that ten of those fifteen will be a three or better. dropping some dice now i get: 2, 3, 1, 1, 2, 2, 1, 4, 6, 2, 4, 6, 6, 6, and 1. more than half my values are less than my target of 3+. is there any way of shortcutting this? more often than not, we're looking for 3+ or 4+ on anywhere from four to thirty-six dice. 15 choose 10 divided by 2^15 for 10 4+'s. You'll need a calculator. In general, if you're making n rolls and you want to know the probability of getting k of some result (with a probability p of getting that result on any single roll, e.g. p=1/2 for a 4+), use the formula (n choose k) * p^k * (1-p)^k. edit: (n choose k) = n! / ((n-k)! * k!), where ! = factorial.
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