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This may be a trivial question, but as a research psychologist I do not have a robust statistics background to answer it.

It appears to me that the likelihood function--$L(\theta | \text{data}) = P(\text{data} | \theta)$--is committing the inverse fallacy which is exactly what using Bayes' theorem avoids. I'm sure that the logic behind the likelihood is sound, but I can't see why this is NOT a case of incorrectly equating two different conditional probabilities (i.e., the inverse fallacy).

kjetil b halvorsen
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ATJ
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  • Specifically, I first came across it in Lynch's (2007) Intro to Applied Bayesian Statistics and Estimation. But it can also be seen on wikipedia: http://en.wikipedia.org/wiki/Likelihood_function and various other sources I've seen. – ATJ Apr 25 '13 at 18:17
  • Your question is answered at http://stats.stackexchange.com/questions/2641/what-is-the-difference-between-likelihood-and-probability (even though it's not exactly a duplicate question). – whuber Apr 25 '13 at 19:15
  • Also have a look at http://stats.stackexchange.com/questions/112451/maximum-likelihood-estimation-mle-in-layman-terms/112480#112480 – kjetil b halvorsen Sep 14 '15 at 14:19

2 Answers2

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From your link:

Confusion of the inverse, also called the conditional probability fallacy or the inverse fallacy, is a logical fallacy whereupon a conditional probability is equivocated with its inverse

i.e. this is talking about making the mistake of thinking P(A|B) is the same as P(B|A).

The likelihood is, however, not regarded as a conditional probability in that sense at all. In other words, $L(\theta|\text{data})$ is clearly understood NOT to be $P(\theta|\text{data})$.

(Indeed, as a function of $\theta$, generally it doesn't even integrate to 1! It can't be a probability distribution in that sense.)

When discussing likelihood in probability terms, people always talk about $P(\text{data}|\theta)$ ... that is, the thing it's defined in terms of.

Given the likelihood $L(\theta|\text{data})$ is not taken to be a conditional probability $P(\theta|\text{data})$, in what sense is this the 'inverse fallacy'?

Glen_b
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  • Thanks. I was making the assumption that L(theta | data) was equivalent to P(theta | data). – ATJ Apr 26 '13 at 00:26
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It's not saying $P(\theta|\text{data})$ = $P (\text{data}|\theta)$. It's defining the likelihood function.