1. Presented by: Karen Miller
Bayes Theorem
Presented by:
Karen Miller
Look for games for class online
Look for pictures of bayes
Picture of laplace

2. Outline Thomas Bayes Published Background Probabilities Bayes Theorem
Applications

3. Thomas Bayes Born in 1702 London, England Little childhood information
Presbyterian Minister
In 1742, elected fellow by the Royal Society of London
Retired in 1752
Died in April of 1761

4. Written Work Only two works published during his life
Divine Benevolence (1731)
Introduction to the Doctrine of Fluxions (1736)
However, he never published his mathematical works
He focused in the areas of probability and statistics
“The probability of any event is the ratio between the value at which an expectation depending on the happening of the event ought to be computed, and the chance of the thing expected upon it’s happening.”

5. Before Bayes, probability was assumed to have a discrete parameter space.
“Bayes invented a new physical model with continuously varying probability of success… He thus gave a geometrical definition of probability as the ratio of two areas.”

6. Publishing of Bayes Theorem
Richard Price examined Bayes’ work after his death
Responsible for the communication to the Royal Society on Baye’s work
An Essay Toward Solving a Problem in the Doctrine of Chances
Intro for essay… spoke highly….

7. An Essay Toward Solving a Problem in the Doctrine of Chances
In this essay one can find
Another version of what becomes Bayes Theorem
Explanation of the binomial distribution
1st section: Conditional Probability
If P (B) > 0, the conditional probability of A given B, denoted by P (A | B), is
P(A | B) =

8. Suppose 70% of students at Saint Joseph’s College pass Core 6, 55% pass Core 8 and 45% percent pass both. If a randomly selected student passed Core 8, what is the probability he or she also passed Core 6?
P(pass 8 | pass 6) = P(pass 8 and 6)/P(pass 6)
= .45 / or 64.29%
We want prob of pass 6 given 8… so prob both / prob pass 6

9. An Essay Toward Solving a Problem in the Doctrine of Chances
The essay demonstrates the estimation of future occurrences of an event, given information of the history of the event.
The probability of future occurrences of an event can be solved using this formula,
A generalization of this formula constructs the present form of Bayes Theorem.

10. Pierre Simon Laplace French mathematician
Responsible for current form of Bayes Theorem
Bayes found the probability that x is between two values given a number of successes and failures
Laplace found an expression for the probability of a number of future successes and future failures given the number of successes and failures.
Didn’t acknowledge bayes right away

11. Bayes – P( | p successes and
In other words,
Bayes – P( | p successes and
q failures )
Laplace – P(m future successes and n future
failures | p successes and q failures )
Richard von Mises states, “We owe Bayes only the statement of the problem and the principle of the solution. The theorem itself was first formulated by Laplace.”

12. Law of Total Probability
Sometimes it is not possible to calculate the probability of an occurrence of an event A however, it may be possible to find P (A | B) and P (A | Bc) for some event B.
Let B be an event with P (B) > 0 and P (Bc) > 0. Then for any event A,
P (A) = P (A | B)P (B) + P (A | Bc)P (Bc)

13. We know that P (A) = P (AB) + P (ABc)
Substitute in the conditional probability
P (AB) = P (A | B)P (B)
P (ABc) = P (A | Bc)P (Bc)
The Law of Total Probability remains
P (A) = P (A | B)P (B) + P (A | Bc)P (Bc)

14. Applying this to the problem of an insurance company that rents 40% of the cars for its customers from agency I and 60% from agency II. If 6% of the cars from agency I and 5% of the cars from agency II break down, what is the probability that a car rented by this company breaks down?
P(car breaks down from the insurance company)
= (.06)(.4) + (.05)(.6)
= or 5.4%

15. Bayes Theorem
Let {B1, B2, …, Bn} be a subset of the sample space S of an experiment.
If for i = 1, 2, …, n, P (Bi) > 0, then for any event A of S with P (A) > 0,
P (Bk | A) =
P(B) is the prior probability and P(B | A) is the posterior probability

16. Proof is very simple
P(A | B) = , conditional probability
Rearranged becomes:
P (AB) = P (B) P (A | B)
P (BA) = P (A) P (B | A)
Therefore P (B)P (A | B) = P (A)P (B | A)
Solve for the P (B | A)
P (B | A) =

17. A box contains seven red and thirteen blue balls
A box contains seven red and thirteen blue balls. Two balls are selected at random and are discarded without their colors being seen. If a third ball is drawn randomly and observed to be red, what is the probability that both of the discarded balls were blue?
Solve P (BB | R) =

18. Applications Classification scale in classroom
A simple classification divides a population in two categories
Mastering
Non-mastering
A test would determine which category a student falls in. The Probabilities can be collected and the result would show mastered skills and others that need attention.

19. Applications Diagnostic testing
Tests identify if a person has a particular disease or not
Tests are not always 100% positive
Bayes Theorem estimates if a person tests positive for a disease if he or she truly has the disease
A cancer is found in 1 in every 2000 people. If a person has the disease there is a 90% the test will result positive. If a person does not have the disease, the test will result in a false positive 1% of the time. The probability that a person with a positive test actually has the cancer is,
P (C | P) =

20. Introduction to Bayesian Statistical Inference
Statistical inference is used to draw conclusions from known data in samples to populations for which data is unknown.
Inference problems are often found in decision problems
There are two different ways to approach probability
Frequentist
Subjective, or Bayesian

21. Frequentist – can be applied to events found through a random process
Frequentists work on problems associated with collective and not individual events.
Bayesian – has no one correct probability
Depends on prior and observed data
Finding the probability of an American man’s height is over 1.75 meters, the result would depend on if we know anything about the man.

22. If no information is known, the probability is based on the proportion of American men taller than 1.75 meters.
Frequentist approach
However, if we have prior knowledge about the man, it must be factor into the probability
If he plays basketball, the probability will be larger than population proportion
Bayesian approach

23. The End