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From |
"Feiveson, Alan H. (JSC-SK311)" <alan.h.feiveson@nasa.gov> |

To |
"statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: RE: probability question |

Date |
Wed, 28 Oct 2009 08:38:16 -0500 |

The reason why I think the retrospective (i.e. finite sampling) approach is needed here is that we are given that it rains (rained?) exactly 7 of the days, not that the probability of rain is 7/120 on any given day. Al Feiveson -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Feiveson, Alan H. (JSC-SK311) Sent: Wednesday, October 28, 2009 8:34 AM To: statalist@hsphsun2.harvard.edu Subject: st: RE: probability question If you look at this retrospectively, you can use the hypergeometric distribution. Suppose we are given that it rained 7 of the 120 days and that the client wore the hat on 4 of those days. The probability that it rained on 3 of the four hat days would then be (7C3 x 113C1 )/(120C4) = .00048146 (I think). whew aCb refers to a things taken b at a time Here "probability" refers to a repeated experiment in which one picks 4 of the 120 days at random to wear the hat, without knowledge of which of those are the rain days. Al Feiveson -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Richard Goldstein Sent: Wednesday, October 28, 2009 7:38 AM To: statalist Subject: st: probability question it's been a long time since I thought about questions like this, but, as a lead-in to a study, a client has asked the following question which he thinks he understands and says is related to where he wants to go: during a consecutive period of 120 days, if it rains on 7 days and my client wears a hat on 4 days (these are independent of any knowledge of the weather), what is the probability that it will rain on 3 of the days on which he is wearing a hat? my client swears that this is not a homework problem for him or his wife or one of their kids! Rich * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: RE: RE: probability question***From:*Jeph Herrin <junk@spandrel.net>

**Re: st: RE: RE: probability question***From:*Richard Goldstein <richgold@ix.netcom.com>

**References**:**st: probability question***From:*Richard Goldstein <richgold@ix.netcom.com>

**st: RE: probability question***From:*"Feiveson, Alan H. (JSC-SK311)" <alan.h.feiveson@nasa.gov>

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