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 January 16th, 2011, 04:23 AM #1 Newbie   Joined: Jan 2011 Posts: 17 Thanks: 0 Stats question [thanks!] Hey so I have a stats question I need a bit of help on...here it is : A store sells HD recorders, assuming that the weekly demand for recorders is a Poisson variable with a mean of 3, find the probability that the store sells more than 20 in a month (4 week period). Now I've tried this question by trying to find the mean amount sold within a month which is 12 (3 x 4) and then used a poisson dist. to find an answer of 0.116, can anyone tell me if this is right or the wrong way of doing it. Also, another part to the same question: Find the minimum number that should be in stock at the beginning of the month so that the store can be at least 95% sure of being able to meet demands of that month. I think you require logs to solve that problem, but I'm not too sure, so I was wondering if anyone could help me find a solution to it. Thanks in advance! January 16th, 2011, 07:51 AM #2 Newbie   Joined: Jan 2011 Posts: 17 Thanks: 0 Re: Stats question [thanks!] Anyone?  January 18th, 2011, 10:11 AM   #3
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Re: Stats question [thanks!]

For the first part, you are correct!
I used Mathematica 8 to check the answer:
Quote:
 Probability[x > 20, x \[Distributed] PoissonDistribution[12.0]]
It returned 0.0115977.

The second part is asking how many items (x) you need to have in stock, so that you will meet the demand (y) with at least 95% probability.

One way to go about is to adding all the possible outcomes until you have a probability greater than 0.95.
Quote:
I've found when you set n=18, the sum equals to 0.962584 < 0.95. So that the minimum number of items that should be in stock is 18.

Alternatively, I used Mathematica to calculate:

Quote:
 prob = Probability[y <= x, y \[Distributed] PoissonDistribution, Assumptions -> x \[Element] Integers && x >= 0] Minimize[x, prob == 0.95 && x > 0, x]
Which gives 17.4536. Tags question, stats Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post Not_a_math_guy Algebra 1 January 2nd, 2013 03:02 PM riotsandravess Advanced Statistics 1 November 18th, 2010 09:59 AM maths_123 Algebra 1 November 1st, 2009 04:22 AM pudro Algebra 0 November 10th, 2008 08:12 PM WhiteHotPro Algebra 1 March 20th, 2008 12:56 PM

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