*To*: debian-user@lists.debian.org*Subject*: Re: backup archive format saved to disk*From*: Andrew Sackville-West <andrew@farwestbilliards.com>*Date*: Thu, 7 Dec 2006 07:59:22 -0800*Message-id*: <[🔎] 20061207155922.GE3909@localhost.localdomain>*Mail-followup-to*: Andrew Sackville-West <andrew@farwestbilliards.com>, debian-user@lists.debian.org*In-reply-to*: <[🔎] 45782813.6030703@sbcglobal.net>*References*: <[🔎] 20061204202016.GB3777@pluto> <45754189.2080705@physik.blm.tu-muenchen.de> <[🔎] 20061205220135.GA1410@pluto> <[🔎] 4576CB1D.300@physik.blm.tu-muenchen.de> <[🔎] 20061206164344.GE30683@localhost.localdomain> <[🔎] 457715CA.1040003@sbcglobal.net> <[🔎] 20061206194450.GK24701@localhost.localdomain> <[🔎] 45782813.6030703@sbcglobal.net>

On Thu, Dec 07, 2006 at 08:41:23AM -0600, Mike McCarty wrote: > Andrew Sackville-West wrote: > >On Wed, Dec 06, 2006 at 01:11:06PM -0600, Mike McCarty wrote: > > > >>Andrew Sackville-West wrote: > > [Mike wrote] > > >>Let p be the probability of failure of each disc, independently of the > >>other. There are four mutually independent events which comprise the > >>space. Both discs may fail [Pr = p^2]. The first disc may fail, while > >>the second does not [Pr = p(1-p)]. The second disc may fail, while the > >>first does not [Pr = (1-p)p]. Both discs may survive [Pr = (1-p)(1-p)]. > >> > >>So, the probability that at least one disc fails is 1-(1-p)(1-p). > >>For p = 0.01, that is 0.0199. > > > > --------------------^^^^ > > > >you aren't using an old pentium are you? ;-) > > If p = 0.01, then (1-p) = 0.99, and (1-p)(1-p) = 0.9801, so that > 1 - (1-p)(1-p) = 0.0199 exactly. that was a joke... > > >>I'll grant you this is not markedly different from 2%, but it is also > >>not simply 2p. > > > > > >huh. been a long time since math class for me. I was under the > >impression that the probability was 2p. chance of one failing is added > >to the chance of the other failing give the chance of both > >failing. But I'll take your word for it :) > > If the probability of the first one failing is 60% and the probability > of the second one failing is 60%, then I suppose you believe that the > probability of one or the other or both failing is 120%. > no I don't. consider me thinking aloud. > The probability in this case works out to 1-(0.4)(0.4) = 0.84. > > The way to think of it is that it is 1 - Pr(none fails). > This extends to any number of independent events. > The problem with trying to add those probabilities is that > the events are not disjoint, so the probabilities do not add. > its starting to come back to me. > You can however decompose it into events A, B, and C which are > disjoint, as A = {disc one fails, but disc two does not}, > B = {disc two fails, but disc A one does not} and > C = {both discs fail}. This is all the ways one or more discs > fail, and the events are disjoint. > > Pr(A) = p(1-p) > Pr(B) = (1-p)p > Pr(C) = p^2 > > So, Pr(at least one disc fails) = p - p^2 + p - p^2 + p^2 > = 2p - p^2 > > This technique gets more complicated as the number of events > increases. > > NB: 1 - (1-p)^2 = 1 - (1 - 2p + p^2) = 2p - p^2, as computed > above. doesn't this involve pascal's triangle somewhere? my brain is too fuzzy for this stuff. I need to get a good book and retrain my brain. thanks for the refresher! A

**Attachment:
signature.asc**

**Follow-Ups**:**Re: backup archive format saved to disk***From:*Mike McCarty <Mike.McCarty@sbcglobal.net>

**References**:**backup archive format saved to disk***From:*Douglas Tutty <dtutty@porchlight.ca>

**Re: backup archive format saved to disk***From:*Douglas Tutty <dtutty@porchlight.ca>

**Re: backup archive format saved to disk***From:*Johannes Wiedersich <johannes@physik.blm.tu-muenchen.de>

**Re: backup archive format saved to disk***From:*Andrew Sackville-West <andrew@farwestbilliards.com>

**Re: backup archive format saved to disk***From:*Mike McCarty <Mike.McCarty@sbcglobal.net>

**Re: backup archive format saved to disk***From:*Andrew Sackville-West <andrew@farwestbilliards.com>

**Re: backup archive format saved to disk***From:*Mike McCarty <Mike.McCarty@sbcglobal.net>

- Prev by Date:
**Re: How to use network-manager in gnome by non root?** - Next by Date:
**offre d'emploi** - Previous by thread:
**Re: backup archive format saved to disk** - Next by thread:
**Re: backup archive format saved to disk** - Index(es):