## Information Theory, Inference, and Learning Algorithms in J - Ensembles

Unlike in the previous post on 27x27 letter bigrams where we made a joint probability matrix by counting, ensembles are usually defined by a set of conditional and marginal probabilities. To get an intuition for this, let’s write out the simple example given in Example 2.3 (p. 25) in J. But first, here is the definition for an ensemble.

An ensemble X is a triple $(x,A_x,P_x)$ where x is an outcome taking on values from $A_x = \{a_1, …, a_I\}$, with associated probabilities $P_x = \{p_1, …, p_I\}$

A joint ensemble XY is an ensemble where each outcome is an order pair (x,y) (also written xy), where $x \in A_x = \{a_1,…,a_I\}$, $y \in A_y = \{b_1,…,b_J\}$, and $P(x,y)$ is called the joint probability of x and y.

(Mackay 03)

Now for the example:

Jo wakes up not feeling well and the doctor orders a test for a disease. The test is 95% reliable, and 1% of Jo’s age and background have the disease. If the test is positive, what is the probability Jo has the disease?

If we define variables disease and test as

• disease=0 => Jo doesn’t have the disease
• disease=1 => Jo has the disease
• test=0 => the test is negative for the disease
• test=1 => the test is positive for the disease

then the probabilities given are

• $P(test=0 | disease=0) = 5\%$
• $P(test=1 | disease=1) = 95\%$
• $P(disease=0) = 99\%$
• $P(disease=1) = 1\%$

To start, we represent $P(test=j|disease=i) = P_{i,j}$ as a matrix ptest_disease where rows represent disease and columns represent test:

and the marginal probability $P(disease=i)$ as a vector

Then we can compute the joint probability by multiplying the two, since $P(test,disease) = P(test|disease) P(disease)$

Now that we have the joint probability, we can calculate any probability that we are interested in. To answer the original question, what is $P(disease=1|test=1)$, we divide each column of joint by it’s sum, since $P(disease|test) = \frac{P(disease,test)}{P(disease)}$

and we see that $P(disease=1|test=1)$ is 16%. So even though the test is 95% accurate, because it’s a rare disease it’s more likely the test is giving a false positive than Jo has the disease.

References