How do 20 questions AI algorithms work?

Question

Simple online games of 20 questions powered by an eerily accurate AI.

How do they guess so well?

Answer 1

You can think of it as the Binary Search Algorithm. In each iteration, we ask a question, which should eliminate roughly half of the possible word choices. If there are total of N words, then we can expect to get an answer after log2(N) questions.

With 20 question, we should optimally be able to find a word among 2^20 = 1 million words.

One easy way to eliminate outliers (wrong answers) would be to probably use something like RANSAC. This would mean, instead of taking into account all questions which have been answered, you randomly pick a smaller subset, which is enough to give you a single answer. Now you repeat that a few times with different random subset of questions, till you see that most of the time, you are getting the same result. you then know you have the right answer.

Of course this is just one way of many ways of solving this problem.

Answer 2

I recommend reading about the game here: http://en.wikipedia.org/wiki/Twenty_Questions

In particular the Computers section:

The game suggests that the information (as measured by Shannon's entropy statistic) required to identify an arbitrary object is about 20 bits. The game is often used as an example when teaching people about information theory. Mathematically, if each question is structured to eliminate half the objects, 20 questions will allow the questioner to distinguish between 2²⁰ or 1,048,576 subjects. Accordingly, the most effective strategy for Twenty Questions is to ask questions that will split the field of remaining possibilities roughly in half each time. The process is analogous to a binary search algorithm in computer science.