Introduction to Probability, Ch#1 notes

A probabilistic model is a mathematical description of an uncertain situation. Every probabilistic model involves an underlying process, called the experiment, that will produce exactly one out of several possible (mutually exclusive)outcomes. The set of *all* possible outcomes is called the sample space of the experiment, and is denoted by $\Omega$. A subset of the sample space, a collection of possible outcomes, is called an event.

The probability law assigns to any event A, a nonnegative P(A)(called the probability of A) that encodes our knowledge or belief about the collective "likelihood" of the elements of A. The probability law *must* satisfy following probability axioms

Nonnegativity: P(A) $\geq$ 0, for every event A.

Additivity: If A and B are two disjoint events, then the probability of their union satisfies, $P(A \cup B)$ = P(A) + P(B) . It can be generalized to any number of disjoint events.

Normalization: P($\Omega$) = 1

-- Conditional Probability --

The conditional probability of an even A, given an event B with P(B) > 0, is defined by P(A|B) = $\frac{P(A \cap B)}{P(B)}$ and specifies a new (conditional) probability law on the same sample space $\Omega$. And, All the probability axioms remain valid on the conditional probability law.

Multiplication Rule:

Assuming that all of the conditioning events have positive probability, we have

P($\cap_{i = 1}^{n} A_i$) = $P(A_1)P(A_2|A_1)P(A_3|A_1 \cap A_2)...P(A_n|\cap_{i = 1}^{n-1} A_i)$

Total Probability Theorem:

Let $A_1, A_2, ..., A_n$ be disjoint events that form a partition of the sample space and assume that probability of all these events is greater than 0. Then, for any event B, we have

P(B) = $P(A_1)P(B|A_1)$ + $P(A_2)P(B|A_2)$ + ... + $P(A_n)P(B|A_n)$

Bayes' Rule:

Let $A_1, A_2, ..., A_n$ be disjoint events that form a partition of the sample space and assume that probability of all these events is greater than 0. Then, for any even B such that P(B) > 0, we have

$P(A_i|B)$ = $\frac{P(A_i)P(B|A_i)}{\sum_{i = 1}^{n}P(A_i)P(B|A_i)}$

-- Independence: --

Two evens A and B are independent if and only if, $P(A \cap B)$ = P(A)P(B). This can be generalized to any number of elements.

Also, as a consequence P(A|B) = P(A), provided P(B) > 0

Conditional Independence:

Two events A and B are said to be conditionally independent given another event C with P(C) > 0, if

$P(A \cap B| C)$ = P(A|C)P(B|C).

And, if in addition $P(B \cap C)$ > 0, then conditional independence is equivalent to the condition

P(A |$B \cap C$) = P(A|C)

Note that, independence does not imply conditional independence and viceversa

## Thursday, September 16, 2010

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