# Bayes optimal classifier vs Likelihood Ratio

I am getting slightly confused by all the probabilistic classifiers.

1. The bayes optimal classifier is given as $max (p(x|C)p(C))$ and if all classes have equal prior then it reduces to $max (p(x|C))$

2. The likelihood ratio is given as $\frac{p(x|C1)}{p(x|C2)}$

If I only have 2 classes with equal prior then what is the difference between the bayes optimal classifier and the likelihood ratio ? Wont they both return me the same class as the output ?

• They are totally different things, so could you clarify what makes you consider them "essentially the same"? – Tim May 24 '16 at 19:21
• Sorry I have edited my question. I hope my question is clearer now. – RuiQi May 25 '16 at 1:37
• What you describe seems to be a Bayes classifier, not a Bayes optimal classifier. – Peter Feb 17 '19 at 10:47

They are not the same, but in you case they could be used for the same purpose.

$$\DeclareMathOperator*{\argmax}{arg\,max} \argmax_{c \in C} p(c|X)$$

i.e., among all hypotheses, take the $c$ that maximizes the posterior probability. You use Bayes theorem

$$\underbrace{p(c|X)}_{\text{posterior}} \propto \underbrace{p(X|c)}_{\text{likelihood}} \underbrace{p(c)}_{\text{prior}}$$

but since using uniform prior (all $c$ are equally likely, so $p(c) \propto 1$) it reduces to the likelihood function

$$p(c|X) \propto p(X|c)$$

The difference between maximizing the likelihood function and comparing the likelihood ratios, is that with likelihood ratio you compare only two likelihoods, while in maximizing the likelihood you may consider multiple hypothesis. So if you have only two hypotheses, then they will do essentially the same thing. However imagine that you had multiple classes, in such case comparing each of them with all the others pair by pair would be a really inefficient way to go.

Notice that likelihood ratio serves also other purpose than finding which of the two models has greater likelihood. Likelihood ratio can be used for hypothesis testing and it tells you how much more (or less) likely is is one of the models comparing to the other. Moreover, you can do the same when comparing the posterior distributions by using Bayes factor in similar fashion.

• Thank you ! I was planning to edit my question to ask about the maximum likelihood estimation since it looks similar to the bayes classifier ! Thanks for clearing my doubt ! – RuiQi May 25 '16 at 6:35