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Subject: "Statistical estimation question"     Previous Topic | Next Topic
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Conferences The CTK Exchange College math Topic #684
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Ralph
guest
Jun-27-08, 11:26 AM (EST)
 
"Statistical estimation question"
 
   Suppose I am sampling from one of two distributions. I know that, given a distribution, the samples are drawn in an independent way. One of the possible distributions is uniform and another called F. Suppose that f (the density of F) is increasing so that these distributions have the monotone likelihood ratio property.

I do not know which of the two distributions is being sampled. My goal is to determine this distribution. I have a prior belief that F is the distribution that is being sampled. This prior is called p.

According to Bayes Rule, given a sample of length N, (s1, s2, s3,...,sN), the probability that I am sampling from distribution F, is given by

(p*f(s1)*f(s2)*...*f(sN))/{p*f(s1)*f(s2)*...*f(sN) (1-p)}

Intuitively it'seems that, as N approaches infinity, this quantity should approach one with probability one if the sample is being drawn from F and 0 with probability 1 if the sample is being drawn from the uniform.

I am a little stumped on how to prove this. Can anyone offer some help or a reference?

Many thanks.


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alexb
Charter Member
2242 posts
Jun-27-08, 11:48 PM (EST)
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2. "RE: Statistical estimation question"
In response to message #0
 
   (p*f(s1)*f(s2)*...*f(sN))/{p*f(s1)*f(s2)*...*f(sN) + (1-p)}
Something is missing here. The other distribution may be uniform but this would no mean that it is uniformly 1. Assuming both are discrete, the above expression should probably look like

(p*f(s1)*f(s2)*...*f(sN))/{p*f(s1)*f(s2)*...*f(sN) + aN(1-p)}

where 0 < a < 1. Denote this P. Then

1/P = 1 + (1 - p)/p · aN / f(s1)·f(s2)·...·f(sN))

If you assume that the sample is distributed according to f, there is a better chance for f(s) to be above a, i.e. nearer the right end where f exceeds uniform density. In which case, indeed, I would expect the second addend to fo to 0 as N grows. On the other hand, if the sample is distributed uniformly on some interval A, B (so that a = 1/(B - A), give or take ...) and f is, say, a straight line equal a at the midpoint (A+B)/2, then for two picks at the same distance from the midpoint but on different sides would be f1 = a - x, f2 = a + x so that

a² > a² - x² = (a - x)(a + x)

and I would expected the second addend grow without bound. For, as N grows, there would be a high probability of being able to combine the samepls into pairs, more or less on the same distance from (A+B)/2. And then you would have a growing number of factors

a² / (a² - x²) > 1

How to do that formally I am afraid I do not know.



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Ralph
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Jul-01-08, 02:21 PM (EST)
 
3. "RE: Statistical estimation question"
In response to message #2
 
   Thank you for the reply Alex.

I apologize for the ambiguity, when I said Uniform I meant U<0,1>. I have simplified the model I was working on to simplify this issue.


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