# LECTURE 4. Conditional Probability and Bayes Theorem

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1 LECTURE 4 Conditional Probability and Bayes Theorem 1 The conditional sample space Physical motivation for the formal mathematical theory 1. Roll a fair die once so the sample space S is given by S {1, 2, 3, 4, 5, 6}. Let so Now what about A 6 appears B an even number appears, P (A) 1 6 P (A) 1 2. P (6 appears given an even number appears). The above probability will be written P (A B) to be read P (A given B). How do we compute this probability? In fact we haven t defined it but we will compute it for this case and a few more cases using our intuitive sense of what it ought to be and then arrive at the definition later. Now we know an even number occurred so the sample space changes, { } 1, 2, 3, 4, 5, 6 so there are only 3 possible outcomes given an even number occurred so P (6 given an even number occurred) 1 3. The new sample space is called the conditional sample space.

2 A very important example 2. Suppose you deal two cards (in the usual way without replacement). What is P ( ) i.e., P (two hearts in a row). Well, Now, what about the second heart? Many of you will come up with and P (first heart) P ( ) ( ) ( ) There are TWO theoretical points hidden in the formula. Let s first look at ( ) P on 2 }{{ nd 12 } 51. this isn t really correct What we really computed was the conditional probability P ( on 2 nd deal on 1 st deal ) Why? Given we got a heart on the first deal the conditional sample space is the new deck with 51 cards and 12 hearts so we get P ( on 2 nd on 1 st) (1) The second theoretical point we used was the formula which we will justify formally later P ( ) P ( on 1 st) P ( ( ) ( ) on 2 nd on 1 st) Two Basic Motivational Problems I will use the next two problems to motivate the theory we will develop in the rest of this lecture. One way to solve these two problems will be contained in the HW Problem which follows this discussion. Another way is to use the general theory we are going to develop in the rest of this lecture.

3 1. What is P ( on 1 st on 2 nd). This is the reverse order of the conditional probability considered in Equation 1 where the answer was intuitively clear but for this reversed order it is not at all clear any more what the answer it. 2. What is P ( on 2 nd with no information on what happened on the 1 st). An argument that works but I cannot justify Here is a proof that will prove that this probability is the same as P ( on the 1 st), namely 13/52 1/4. First you know a card has been played so at first guess you might say 13/51 but that card could have been a heart with probability 1/4 so we subtract 1/4 from the numerator to get [13 (1/4)]/51 51 (4)(51) 1/4 as I claimed. Intuitively there are 51 cards available and 12 and 3/4 hearts left. I know of no way to justify this rigorously - the sum amounts to adding a probability to a cardinality which I do not know how to justify. I think the sum is an expected value (to be defined later), the expected number of hearts left. However at the moment I do not see why the probability should be the ratio of this expected value divided by the number of cards available. Later in this Lecture and on the Homework problem that follows there will be rigorous proofs. The Formal Mathematical Theory of Conditional Probability S A A intersect B B Figure 1: Conditional Probability Picture #(S) n, #(A) a, #(B) b, #(A B) c.

4 Let S be a finite set with the equally-likely probability measure and A and B and A B be the events with the cardinalities shown in the picture. Problem Compute P (A B). We are given B occurs so the conditional sample space is B. Once again we will compute a conditional probability without the formal mathematical definition of what it is. Only part of A is allowed since we know B occurred namely A B P (A B) #(A B) #(B) c b. We can rewrite this as so P (A B) c b c/n b/n P (A B). P (B). (*) P (B) This intuitive formula for the equally likely probability measure leads to the following.

5 The Formal Mathematical Definition Let A and B be any two events in a sample space S with P (B) 0. The conditional probability of A given B is written P (A B) and is defined by P (A B). (*) P (B) So if P (A) 0 then since A B B A. P (B A) P (B A) P (A) P (A) (**) We won t prove the next theorem but you could do it and it is useful. Theorem. Fix B with P (B) 0. P ( B) satisfies the axioms (and theorems) of a probability measure see Lecture 1. For example 1. P (A 1 A 2 B) P (A 1 B) + P (A 2 B) P (A 1 A 2 B) 2. P (A B) 1 P (A B) P (A ) does not satisfy the axioms and theorems. The Multiplicative Rule for Rewrite (**) as P (A)P (B A) (#) (#) is very important, more important than (**). It complements the formula P (A B) P (A) + P (B) P (A B). Now we know how P interacts with both of the basic binary operations and. We will see that if A and B are independent then I remember the above formula # as Hopefully this will help you remember it. More generally P (A)P (B). P (first second) P (first)p (second first). P (A B C) P (A)P (B A)P (C A B).

6 Exercise Write down P (A B C D). Traditional Example An urn contains 5 white chips, 4 black chips and 3 red chips. Four chips are drawn sequentially without replacement. Find P (W RW B). S {5W, 4B, 3R}. Solution What did we do formally P (W RW B) ( ) ( ) ( ) ( ) P (W RW B) P (W ) P (R W ) P (W W R) P (B W R W ). Bayes Theorem (pg.762) Bayes Theorem is a truly remarkable theorem. It tells you how to compute P (A B) if you know P (B A) and a few often things. For example we will get a second way (aside from the Problem at the end of this Lecture) to compute our favorite probability P ( on 1 st on 2 nd ) because we know the reversed probability P ( on 2 nd on 1 st ) 12/51 First we will need a preliminary result. The Law of Total Probability Let A 1, A 2,, A k, be mutually exclusive (A i A j, i j) and exhaustive A 1 A 2 A k S the whole space). Then for any event B P (B) P (B A 1 )P (A 1 ) + P (B A 2 )P (A 2 ) + + P (B A k )P (A k ). ( ) Special Case k 2 so we have A and A P (B) P (B A)(P (A) + P (B A )P (A ). ( ) Application to our ssecond basic problem Now we can prove Put P ( on 2 nd with no other information) B on 2 nd A on 1 st A on 1 st

7 Here we write for nonheart. So Now P ( on 1 st ) P ( on 2 nd on 1 st ) Add fractions: Factor out 13 P (B) P (B A)P (A) + P (B A )P (A) P ( on 2 nd on 1 st )P ( on 1 st ) +P ( on 2 nd on 1 st )P ( on 1 st ) ( ) ( ) ( ) ( ) (13)( ) (51)(52) (12)(13) + (13)(39) (51)(52) (13) (52) (13) (52) Done! ( ) is easy to prove but we won t do it. I did it in class for k 4. Now we can state Bayes Theorem.

8 Bayes Theorem Let A 1, A 2,, A k be a collection of k mutually exclusive and exhaustive events. Then for any event B with P (B) > 0 we have P (A j B) P (B A j )P (A j ) k i1 P (B A i)p (A j ) Again we won t give the proof for general k. For most applications k 2 and in fact we don t the complicated formula for the denominator on the right-hand side (in fact it is just P (B) which we can usually easily compute in applications). Special Case k 2 Suppose we have two events A and B with P (B > 0). then P (A B) P (B A)(P (A) P (B A)P (A) + P (B A )P (A ). There is a sometimes better way to look at this formula. Namely P (A B) P (A) P (B) P (B A). The two equations are equivalent because of the Law of Total Probability Equation. I will prove this case in the second form above because it shows what is really going on. The point is that is commutative so A B B A. We will apply P to both sides of this equation and solve for P (A B) and thereby prove Bayes Theorem. Indeed P (A)P (B A) P (B)P (A B) and solving for P (A B) P (A B) P (A) P (B) P (B A). P (B A) so Application to our first basic problem We return to the computation of P ( on 1 st on 2 nd ). We will use Bayes Theorem. This is the obvious way to do it since we know the probability the other way around Put A ( on 1 st ) and B ( on 2 nd ).

9 By Bayes Theorem P (A B) P (B) P ( ) P ( on 2 nd with no other information) Now we know from earlier in this lecture that ( ) ( ) P ( ) and we saw just before the proof of Bayes Theorem (the solution of our first basic problem) that P ( on 2 nd with no other information) Hence P ( on 1 st on 2 nd ) P ( on 2nd on 1 st )!! P ( on 1 st on 2 nd ). This completes the solution of the second basic problem. Compulsory Reading (for your own health). In case you or someone you love test positive for a rare (this is the point) disease, read Example 2.30, pg. 73. Misleading (and even bad) statistics is rampant in medicine. Problems Problem 1 The point of this problem is to compute P ( on1 st on2 nd ) and P ( on1 st ) using an abstract approach counting pairs. (i). Let S be the set of unordered pairs of distinct cards. S will be our sample space and the previous hard to understand problems will be solved by counting subsets of pairs with various conditions on one or both elements of the pair. Compute #(S). (ii). Let A be the subset of S consisting of pairs where both cards in the pair are hearts so Compute A { } S. #(A) and P (A #(A) #(S). (iii) Let B be the subset of all pairs to that it the second card in the pair is a heart and the first card is anything. Compute #(B) and P (B) solving 2. above (namely the probability that the second card is a heart with no condition on the first). (iv) Compute P ( on 1 st on 2 nd) by taking the ratio P (A)/P (B), see the defining formula (*) two pages later, or else by computing within the conditional sample space S B which is a subspace of the set of all pairs S.

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