1 introduction

1.1 sample space

Denoted by \Omega is the set of all possible outcomes of an experiment.

1.2 event

\omega \subset \Omega - events

1.2.1 simple

Event is called simple when |\omega| = 1

1.2.2 compound

Event is called compound when |\omega| > 1

1.3 Venn diagrams

https://mathworld.wolfram.com/images/eps-gif/VennDiagram_900.gif

1.4 properties

Where A, B, C are events and \bar A denotes the complement of A

1.5 axioms of probability

1.5.1 propositions

1.6 equally likely outcomes

P(A) = \frac{N(A)}{N}

1.7 conditional probability

P(A|B) = \frac{P(A \cap B)}{P(B)}

1.7.1 multiplication rule

P(A \cap B) = P(A|B) \cdot P(B)

1.7.2 total probability

Events A_1, A_2, \cdots are exhaustive when \bigcup A_i = \Omega

When such events are exhaustive and mutually exclusive then: P(B) = \sum P(B \cap A_i) = \sum P(A_i) \cdot P(B | A_i)

1.7.3 Bayes’ formula

Let A_1, A_2, \cdots be mutually exclusive exhaustive events. Then for any other event B and all j:

P(A|B) = \frac{P(B|A)\cdot P(A)}{P(B)}

or

P(A_j|B) = \frac{P(B|A_j) \cdot P(A_j)}{\sum P(B|A_i) \cdot P(A_i)}

Also:

P_B(A) = P(A|B)

1.8 independence

When P(A \cap B) = P(A)P(B) then A and B are independent events

If A and B are independent then so are the complements:

1.8.1 more than two events

For A_1, A_2, \cdots A_n

P(\bigcap A_i) = \prod P(A_i) for all combinations of A sets

Any combinations of complements are also mutually independent

1.8.2 inclusion exclusion

P(\bigcup A_i) = 1 - \prod (1 - P(A_i))

1.8.3 Bernoulli trials

Let an experiment have only 2 possible outcomes: failure (f) or success (s) where P(s) = p and P(f) = 1 - p = q

The probability that we obtain exactly k successes in the n independent Bernoulli trials is b(k; n, p) = \binom{n}{k}p^k(1-p)^{n-k}

1.8.3.1 Poisson lemma

lim_{n\to \infty}n\cdot p_n = \lambda > 0 then

lim_{n\to\infty}\binom{n}{k}p_n^k(1-p_n)^{n-k} = e^{-\lambda}\cdot\frac{\lambda^k}{k!}

F(k; \lambda) = \sum_{i=0}^k e^{-\lambda} \frac{\lambda^i}{i!}