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Difference between revisions of "Probability"

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'''Probability''' is traditionally considered one of the most difficult areas of [[mathematics]], since probabilistic arguments often come up with apparently paradoxical or counterintuitive results. Examples include the [[Monty Hall paradox]] and the [[birthday problem]].  Probability can be loosely defined as the chance that an event will happen.
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'''Probability''' is one of the most difficult areas of [[mathematics]] to define, explain, or understand.  Probability can be loosely defined as the chance that an event will happen.
 
  
  
 +
==Video For Beginners!==
 +
https://youtu.be/OOdK-nOzaII?t=979
  
 
== Introductory Probability ==
 
== Introductory Probability ==
 
Before reading about the following topics, a student learning about probability should learn about introductory [[counting]] techniques.
 
Before reading about the following topics, a student learning about probability should learn about introductory [[counting]] techniques.
−
* [[dependent probability]]
+
* dependent probability
−
* [[independent probability]]
+
* independent probability
 +
 
 +
== Formal Definition of Probability ==
 +
The foundations of probability reside in an area of [[analysis]] known as [[measure theory]]. Measure theory in general deals with [[integral|integration]], in particular, how to define and extend the notion of "area" or "volume." Intuitively, therefore, probability could be said to consider how much "volume" an event takes up in a space of outcomes. Measure theory does assume considerable mathematical maturity, so it is usually ignored until one reaches an advanced undergraduate level. Once measure theory is covered, probability does become a lot easier to use and understand.
 +
 
 +
In the language of measure theory, probability is formally defined as a triple known as a [[probability space]], denoted <math>(\Omega, \mathfrak{a}, \mathit{P})</math>. Here <math>\Omega</math> is a set called the sample space, and <math>\mathfrak{a}</math> is a class of events given by certain subsets of <math>\Omega</math>. <math>\mathfrak{a}</math> must satisfy certain properties (it must be a [[sigma-algebra|<math>\sigma</math>-algebra]]) to qualify as a class of events. Together, <math>\Omega</math> and <math>\mathfrak{a}</math> form what is known as a [[measurable space]], <math>(\Omega, \mathfrak{a})</math>. <math>\mathit{P}:\mathfrak{a}\to [0,1]</math> is an assignment with certain properties (it is a special kind of [[measure]]), called the probability function, or probability measure. It assigns a "volume" to each possible event.
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As a simple example, consider a single coin-toss. In this case, <math>\Omega = \{H, T\}</math>, <math>\mathfrak{a} = \{\emptyset, \{H\}, \{T\}, \{H, T\}\}</math>, and <math>\mathit{P}</math> assigns the following probabilities to events in <math>\mathfrak{a}</math>:
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<math>\mathit{P}(\emptyset)=0</math>,
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<math>\mathit{P}(\{H\})=0.5</math>,
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<math>\mathit{P}(\{T\})=0.5</math>,
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<math>\mathit{P}(\{H, T\})=1</math>.
 +
 
 +
We can interpret this as saying that the event of getting Heads, and the event of getting Tails, each take up an equal half of the set of possible outcomes; the event of getting Heads or Tails is certain, and likewise the event of getting neither Heads nor Tails has probability 0.
 +
 
 +
Of course, to understand this example doesn't need measure theory, but it does show how to translate a very basic situation into measure-theoretic language. Furthermore, if one wanted to determine whether the coin was fair or weighted, it would be difficult to do that without using inferential methods derived from measure theory.
 +
 
  
 
=== Types of Probability ===
 
=== Types of Probability ===
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* [[combinatorial probability]] problems involve counting outcomes.
 
* [[combinatorial probability]] problems involve counting outcomes.
 
* [[geometric probability]]
 
* [[geometric probability]]
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 +
Important subdivisions of probability include
 +
* [[stochastic processes]]
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* [[mathematical statistics]]
  
 
=== Example Problems ===
 
=== Example Problems ===
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==== Intermediate ====
 
==== Intermediate ====
 
* [[2006_AIME_II_Problems/Problem_5 | 2006 AIME II Problem 5]]
 
* [[2006_AIME_II_Problems/Problem_5 | 2006 AIME II Problem 5]]
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* [[2007_AIME_II_Problems/Problem_10 | 2007 AIME II Problem 10]]
  
 
== Resources ==
 
== Resources ==
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−
* [http://www.artofproblemsolving.com/Resources/probarticle.pdf Three Types of Probability]
 
  
 
* [http://www.artofproblemsolving.com/Books/AoPS_B_Item.php?item_id=202 Introduction to Counting and Probability] by [[David Patrick]]
 
* [http://www.artofproblemsolving.com/Books/AoPS_B_Item.php?item_id=202 Introduction to Counting and Probability] by [[David Patrick]]
  
 
* [http://www.artofproblemsolving.com/Books/AoPS_B_Item.php?item_id=302 Intermediate Counting and Probability] by [[David Patrick]]
 
* [http://www.artofproblemsolving.com/Books/AoPS_B_Item.php?item_id=302 Intermediate Counting and Probability] by [[David Patrick]]
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[[Category:Combinatorics]]
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[[Category:Mathematics]]

Revision as of 11:47, 2 October 2022

Probability is traditionally considered one of the most difficult areas of mathematics, since probabilistic arguments often come up with apparently paradoxical or counterintuitive results. Examples include the Monty Hall paradox and the birthday problem. Probability can be loosely defined as the chance that an event will happen.


Video For Beginners!

https://youtu.be/OOdK-nOzaII?t=979

Introductory Probability

Before reading about the following topics, a student learning about probability should learn about introductory counting techniques.

  • dependent probability
  • independent probability

Formal Definition of Probability

The foundations of probability reside in an area of analysis known as measure theory. Measure theory in general deals with integration, in particular, how to define and extend the notion of "area" or "volume." Intuitively, therefore, probability could be said to consider how much "volume" an event takes up in a space of outcomes. Measure theory does assume considerable mathematical maturity, so it is usually ignored until one reaches an advanced undergraduate level. Once measure theory is covered, probability does become a lot easier to use and understand.

In the language of measure theory, probability is formally defined as a triple known as a probability space, denoted $(\Omega, \mathfrak{a}, \mathit{P})$. Here $\Omega$ is a set called the sample space, and $\mathfrak{a}$ is a class of events given by certain subsets of $\Omega$. $\mathfrak{a}$ must satisfy certain properties (it must be a $\sigma$-algebra) to qualify as a class of events. Together, $\Omega$ and $\mathfrak{a}$ form what is known as a measurable space, $(\Omega, \mathfrak{a})$. $\mathit{P}:\mathfrak{a}\to [0,1]$ is an assignment with certain properties (it is a special kind of measure), called the probability function, or probability measure. It assigns a "volume" to each possible event.

As a simple example, consider a single coin-toss. In this case, $\Omega = \{H, T\}$, $\mathfrak{a} = \{\emptyset, \{H\}, \{T\}, \{H, T\}\}$, and $\mathit{P}$ assigns the following probabilities to events in $\mathfrak{a}$: $\mathit{P}(\emptyset)=0$,

$\mathit{P}(\{H\})=0.5$,

$\mathit{P}(\{T\})=0.5$,

$\mathit{P}(\{H, T\})=1$.

We can interpret this as saying that the event of getting Heads, and the event of getting Tails, each take up an equal half of the set of possible outcomes; the event of getting Heads or Tails is certain, and likewise the event of getting neither Heads nor Tails has probability 0.

Of course, to understand this example doesn't need measure theory, but it does show how to translate a very basic situation into measure-theoretic language. Furthermore, if one wanted to determine whether the coin was fair or weighted, it would be difficult to do that without using inferential methods derived from measure theory.


Types of Probability

Part of a comprehensive understanding of basic probability includes an understanding of the differences between different kinds of probability problems.

Important subdivisions of probability include

Example Problems

Introductory

Intermediate

Resources

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