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From "Quiroz Hernandez, Andres" <Andres.QuirozHernan...@xerox.com>
Subject RE: Probability from log likelihood in LDA output
Date Mon, 06 Dec 2010 20:06:08 GMT
```Thanks for your quick reply, Ted. It looks like either the probabilities are not normalized
or the function being used is not a simple sum of log probabilities, because exp does not
always return a value between 0 and 1. I will take a look at the code to see if I can find
exactly how the value is calculated (but if anyone knows the function used, and if I can directly
invert it to find P(w|t) please let me know).

Thanks again,

Andres

-----Original Message-----
From: Ted Dunning [mailto:ted.dunning@gmail.com]
Sent: Monday, December 06, 2010 11:57 AM
To: user@mahout.apache.org
Subject: Re: Probability from log likelihood in LDA output

Yes.  I should be possible to use exp to get the actual probability.  The
fact that it is a sum
of log probabilities just means that the probability is a product of
probabilities.

It is possible that the probabilities are not normalized, but that would be
a bit surprising for
this kind of algorithm.

On Mon, Dec 6, 2010 at 8:02 AM, Quiroz Hernandez, Andres <
Andres.QuirozHernandez@xerox.com> wrote:

> Hello,
>
> As I understand it, the output for LDA is a log likelihood value for
> each word/topic pair, which is a function of log(P(w|t)). Is it possible
> to invert that function to obtain P(w|t)? I have a feeling it is not,
> since it looks like the final value is obtained as a sum of log
> probabilities, but I just wanted to check, since an output as a
> probability is more readable than the likelihood value given.
>
> Thanks,
>
> Andres
>
```
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