What is the difference between a Generative and Discriminative Algorithm?

请帮助我理解生成式算法和区分算法之间的区别，同时牢记我只是一个初学者。

---

Let's say you have input data x and you want to classify the data into labels y. A generative model learns the joint probability distribution p(x,y) and a discriminative model learns the conditional probability distribution p(y|x) - which you should read as "the probability of y given x".

Here's a really simple example. Suppose you have the following data in the form (x,y):

(1,0), (1,0), (2,0), (2, 1)

p(x,y) is

      y=0   y=1
     -----------
x=1 | 1/2   0
x=2 | 1/4   1/4

p(y|x) is

      y=0   y=1
     -----------
x=1 | 1     0
x=2 | 1/2   1/2

If you take a few minutes to stare at those two matrices, you will understand the difference between the two probability distributions.

The distribution p(y|x) is the natural distribution for classifying a given example x into a class y , which is why algorithms that model this directly are called discriminative algorithms. Generative algorithms model p(x,y) , which can be tranformed into p(y|x) by applying Bayes rule and then used for classification. However, the distribution p(x,y) can also be used for other purposes. For example you could use p(x,y) to generate likely (x,y) pairs.

From the description above you might be thinking that generative models are more generally useful and therefore better, but it's not as simple as that. This paper is a very popular reference on the subject of discriminative vs. generative classifiers, but it's pretty heavy going. The overall gist is that discriminative models generally outperform generative models in classification tasks.

---

A generative algorithm models how the data was generated in order to categorize a signal. It asks the question: based on my generation assumptions, which category is most likely to generate this signal?

A discriminative algorithm does not care about how the data was generated, it simply categorizes a given signal.

---

Imagine your task is to classify a speech to a language:

you can do it either by:

1) Learning each language and then classifying it using the knowledge you just gained

OR

2) Determining the difference in the linguistic models without learning the languages and then classifying the speech.

the first one is the Generative Approach and the second one is the Discriminative approach.

check this reference for more details: http://www.cedar.buffalo.edu/~srihari/CSE574/Discriminative-Generative.pdf

链接地址: http://www.djcxy.com/p/6794.html

上一篇: 大Ө符号代表什么？

下一篇: 生成式和判别式算法有什么区别？