Intersection | Snitt | $\cap$

The intersection of two events $A$ and $B$, denoted by the symbol $A\cap B$, is the event containing all elements that are common to both $A$ and $B$. I.e. all the elements that are both in $A$ and $B$.

If you're familiar with logic, think of $\cap$ as the logical operator "and" ($\wedge$) or a conjunction.

Example: Let $D$ be the event that the outcome of a die roll is prime while $E$ is the event that the outcome of said die roll is odd. $D = \{2, 3, 5\}$ and $E = \{1, 3, 5\}$. Then the intersection of $D$ and $E$ is $D\cap E = \{3, 5\}$.

Mutual exclusion | disjoint sets | disjunkte mengder

Two events $A$ and $B$ are mutually exclusive, or disjoint, if $A\cap B = \emptyset$, i.e. if $A$ and $B$ have no elements in common.

Union | $\cup$

The union of two events $A$ and $B$, denoted $A\cup B$, is the event containing all the elements that belong to $A$ or $B$ or both. A trick to remember if $\cup$ or $\cap$ is the symbol for union is that the one representing union looks like the first character in the word "union".

If you are familiar with logic, think of $\cup$ as the logical operator "or" ($\vee$) or a disjunction.

Balanced object | Balansert objekt

An object (e.g. a die or a coin) is balanced if all possible outcomes are equally likely when it is rolled/flipped/thrown.

Discrete sample space | Diskrete utfallsrom

A discrete sample space contains either a finite number of possibilities or an unending sequence with as many elements as there are whole numbers.

Typically represents count data, such as the number of heads after $k$ coin flips or how many children are born on a given day.

Continuous sample space | Kontinuerlig utfallsrom

A continuous sample space contains an infinite number of possibilities equal to the number of points on a line segment. Note that between any two different points on a line segment there are an infinite number of points.

Typically represents measured data, such as height or weight.

Degrees of freedom | Frihetsgrader

"The degrees of freedom is the number of values in the final calculation of a statistic that are free to vary."

It is the quantity $n-1$, where $n$ is the sample size of the experiment.

It could also be the parameter $v$ of a Chi-squared($\chi^{2}$)-distribution. Because that is also referred to as "$v$ degrees of freedom".

Population | Populasjonen | Univers

The population (often of an experiment) is the totality of observations with which we are concerned.

The number of observations in the populations is defined to be the size of the population.

Each observation in a population is a value of a random variable $X$ having some probability distribution $f(x)$.

Sample | Utvalg

A sample is a subset of a population.

Statistic

A statistic is a function of the random variables that constitute a random sample.

Little blue book | Tabeller og formler i statistikk

A little blue book titled "Tabeller og formler i Statistikk", available from Akademika, which one is allowed to bring to this course's exam. Its ISBN is 978-82-519-1595-3.

Note that the one used at NTNU is a 2011-revision by NTNU's IMF institute of the original 2001-edition by Kvaløy, Jan Terje - Tjelmeland, Håkon.

Multiplication rule | Rule of product | Multiplication principle

More information: Wikipedia

"The fundamental principle of counting," states that if an operation can be performed in $n_{1}$ ways, and if for each of these ways a second operation can be performed in $n_{2}$ ways, ..., and if for each of these ways a $k$th operation can be performed in $n_{k}$ ways, then the sequence of $k$ operations can be performed in $n_{1}n_{2}...n_{k}$ ways.

Permutation | Permutasjon | $_{n}P_{r}$

More information: Wikipedia

A permutation is an ordered arrangement of all or part of the members of a set. Example: The set $S = \{a, b\}$ has two permutations: $\{a, b\}$ and $\{b, a\}$.

A set containing $n$ elements/objects has $n!$ possible permutations.

$r$ objects can be selected from a set of $n$ distinct objects in $$_{n}P_{r} = \frac{n!}{(n-r)!}$$ ways.

Example: $2$ objects are selected from the set $S = \{a, b, c\}$. There are $\frac{3!}{(3-2)!} = 6$ possible solutions (permutations): $\{a, b\}$, $\{a, c\}$, $\{b, a\}$, $\{b, c\}$, $\{c, a\}$ and $\{c, b\}$.

Used when/for: Selecting people (distinct objects) from a group/set,

A circular permutation is an circular arrangement of objects. There are $(n-1)!$ circular permutations of $n$ objects. Rotating a circular permutation does not produce a new permutation.

Distinguishing things. If a set consists of $n$ objects of which $n_{1}$ are of one kind, ..., $n_{k}$ are of a $k$th kind, its number of distinct permutations is $$\frac{n!}{n_{1}!...n_{k}}$$ Example: We have $10$ coloured balls, of which $3$ are red, $2$ are green and $5$ are vomit. There are $\frac{10!}{(3!)(2!)(5!)} = 2520$ possible permutations of this example.

Combination | Kombinasjon | $_{n}C_{r}$

More information: Wikipedia

"A combination is an unordered permutation." - Lao Tze-Te, fictional chinese statistician/tea-leaf interpreter.

The number of combinations of $n$ distinct objects taken $r$ at a time is $$_{n}C_{r} = \binom{n}{r} = \frac{n!}{r!(n-r)!}$$ A combination is an unordered permutation. $2$ objects can be selected from the set $\{a, b\}$ Example: Look at the set $\{a, b\}$. It has two permutations ($\{a, b\}$ and $\{b, a\}$) but only there is only one ($1$) possible way to select a combination of size $2$ from it, as $\{a, b\}$ and $\{b, a\}$ are the same combination.

In general, when one is calculating a series of possible combinations (poker hands, anyone?) the order is irrelevant but each selection of $n$ elements removes that many elements from the set ("tilfeldig trekk uten tilbakelegging").

Note: While a combination ignores the elements' ordering, a series of selections from the same source (i.e. a card deck) has to be calculated from most specific to least specific, as the more specific selections tend to be subsets of the least specific selections. For example, any specific combination of $n$ cards is a subset of the unspecified selection of $n$ cards ("any $n$ cards") – the royal straight flush is a subset of the straight and flush).

Example: You want to pull some cards out of a standard issue playing card deck at random without caring about the order. A standard card deck has $52$ distinct cards, each with one of $4$ distinct suits and $13$ distinct ranks. There are $\binom{52}{2} = 1326$ possible combinations of $2$ cards out of $52$ if we don't care about their suit or rank. But what if we did? How many ways can we draw $2$ cards, both of which are jacks (of a specific rank)? Since there's $4$ cards with the same rank, there are $\binom{4}{2} = 6$ combinations of two jacks. In other words, in $6$ out of the $1326$ possible ways to draw two cards, we get two jacks. But what if we just want both cards to have the same rank? There's $13$ different ranks, of which we are interested in either: $\binom{13}{1}\binom{4}{2} = 78$ of the $1326$ possible ways to draw two cards of the same rank (a pair).

What if we wanted both cards to be of the same suit, but didn't care about their rank? $\binom{13}{2}\binom{4}{1} = 312$.

What if we wanted one of the cards to be a spade but didn't care about the other cards' suit? There are $13$ possibilities for one of the cards (since it has to have a specific suit), and $51$ possibilities for the other card (because we don't care about it but it can't be the same card as the other one). So! There are $(13)(51) = 663$ ways (out of $1326$) to end up with at least one spade if you draw two cards.

But what if we want one spade, and one non-spade? $\binom{13}{1}\binom{39}{1} = 741$ possibilities.

Used for/when: Playing cards (poker hands and such), tilfeldig trekk uten tilbakelegging.

Complementary events | Complement

$A$ is an event. $A'$ is all events in which $A$ is not. For two such complementary events then $P(A) + P(A') = 1$. If you are into logic you can think of a complementary event as a negation: $A'$ is all other events than $A$, or simply "not $A$".

Additive rule | addisjonssetningen

If $A$ and $B$ are two events then $P(A\cup B) = P(A) + P(B) - P(A\cap B)$. That is, the probability of either $A$ or $B$ occuring is equal to the sum of the probability of $A$ occuring and the probability of $B$ occuring minus the probability of $A$ AND $B$ occuring. This is because the event that both $A$ and $B$ occurs ($A\cap B$) is a subset of both $A$ and $B$: $(A\cap B) \subset A$ and $(A\cap B) \subset B$ (you could draw a venn diagram to verify this. Nothing fancy, just a box with two intersecting circles). This follows from observing that if both $A$ and $B$ occurs then, well, they both occur.

However if $A$ and $B$ are mutually exclusive (if $A\cap B = \emptyset$) then $P(A\cup B) = P(A) + P(B)$.

Additive rule for three events, $A$, $B$, $C$: $P(A\cup B\cup C) = P(A) + P(B) + P(C) - P(A\cap B) - P(A\cap C) - P(B\cap C) + P(A\cap B\cap C)$

Random Variable | stochastic variable | stokastisk variabel

A random variable is a function that associates a real number with each element in the sample space. Think of it as a variable in an experiment: if we sample a population (e.g. we select a bunch of people from Oslo at random), their height, weight, eye colour and feelings towards fish are all random variables. Anything that can vary between sample points can called a random variable.

In this course, random variables are usually denoted by capital letters (e.g. $X$, $Y$), while the corresponding lower case/small letter (e.g. $x$, $y$) denotes one of its values.

Example: The sample space describing the possible outcomes of flipping a balanced coin -- that is a coin which is equally likely to land as heads or tails when flipped -- is $S = \{TTT,TTH,THT,HTT,THH,HTH,HHT,HHH\}$. We can define $X$ as "the number of heads" when flipping a coin three times, which would cause $X$ to assume either $0$, $1$, $2$ or $3$. One can also say that $X$ takes on all values $x \in \{0, 1, 2, 3\}$. The book seems to do both. If the outcome is $TTT$ then $X$ is $0$. If $X$ is $2$ then the possible outcomes are $\{THH, HTH, HHT\}$.

Discrete Probability Distributions | Diskrete sannsynlighetsfordelinger

"A discrete random variable assumes each of its values with a certain probability."

Example: If we toss a coin three times and denote $X$ as "the number of heads", $X$ assumes the value $1$ with probability $\frac{3}{8}$, because all $8$ sample points (possible outcomes) are equally likely and $3$ of them contains $1$ head.

$$ \begin{array}{c|cccc} x & 0 & 1 & 2 & 3 \\ \hline P(X = x) & \frac{1}{8} & \frac{3}{8} & \frac{3}{8} & \frac{1}{8} \end{array}$$ In the table above, the first row contains the possible values of $x$ while the second row contains the probability that $X$ will assume the given value of $x$. E.g. Row 2, column 2 contains $\frac{1}{8}$: the probability that $X$ (the number of heads in three coin flips) is $0$. Notice how the fractions in the second row add up to $1$, and that $X$ cannot assume a value not featured in the first row?

Continuous Probability Distributions | Kontinuerlig sannsynlighetsfordeling

"A continuous random variable has a probability of $0$ of assuming exactly any of its possible values."

Wait, what? Assume we have a device which allows us to accurately measure the weight of an object in kilograms to twenty decimal places. Further, assume this device is being used to weigh every single adult above $21$ years of age. Because of the device's ridiculous accuracy, the probability that an individual will weigh any given weight is vanishingly small. In fact this probability is so small we declare it to be $0$ -- the probability of selecting an individual at random who weighs, say, exactly $65.00000000000000000000$ kilograms is said to be $0$. So what do we do? We ask about intervals -- what is the probability that a randomly selected individual weighs at least $65$ but not more than $66$ kilograms, for example.

The values of continuous random variables are plotted as a graph. The probability that the random variable assumes some value within a given range is equal to the area beneath the graph within the range.

Joint Probability Distributions | Simultan sannsynlighetsfordeling

Expected value of a random variable | mean | forventningsverdi | $\mu$ | $E(X)$

Also known as: mean of the random variable $X$, mean of the probability distribution of $X$. It is the expected value of $X$. Which means that if the experiment is repeated an infinite number of times, one could expect that $X$ would be, on average, $E(X)$.

Let $X$ be a random variable with probability distribution $f(x)$. The mean, or expected value, of $X$ is $$ \begin{array}{lc} \text{Discrete: } && \mu_x = E(X) = \sum_{x}xf(x) \\ \text{Continuous: } && \mu_x = E(X) = \int_{-\infty}^{\infty}xf(x) \end{array}$$

What if we have a random variable that depends on the random variable $X$? For example a function defined with $X$ as its input: $g(X)$. The expected value of the random variable $g(X)$ is $$ \begin{array}{lc} \text{Discrete: } && \mu_{g(X)} = E[g(X)] = \sum_{x}g(x)f(x) \\ \text{Continuous: } && \mu_{g(X)} = E[g(X)] = \int_{-\infty}^{\infty}g(x)f(x)dx \end{array}$$ As you can see, if $g(X) = X$ then these expressions become equal to the previous ones.

Variance | varianse | $\sigma^{2}$

The variance of the random variable $X$, or the variance of the probability distribution of $X$, is a measure of how far a set of numbers is spread out. It describes how far the numbers lie from the mean/expected value.

The variance of a random variable $X$ is $$\sigma_{x}^{2} = E(X^{2}) - \mu_{X}^{2}$$ That is, the expected value of $X^{2}$ minus the expected value of $X$ squared.

Standard Deviation | standardavvik | $\sigma$

The standard deviation of a random variable $X$ is the positive square root of the variance of $X$. $$\sigma_{X} = \sqrt{\sigma_{X}^{2}}$$

It shows how much variation exists from the mean/expected value. A low standard deviation indicates that the data points tend to be very close to the mean, while a high standard deviation indicates that the data points are spread out over a large range of values.

Covariance | kovarianse | Cov(X,Y)

The covariance of two random variables $X$ and $Y$ with means $\mu_{X}$ and $\mu_{Y}$, respectively, is given by $$Cov(X,Y) = \sigma_{XY} = E(XY) - \mu_{X}\mu_{Y}$$

Statistical indepedence | uavhengighet

$X$ and $Y$ are two random variables with joint probability distribution $f(x,y)$ and marginal distributions $g(x)$ and $h(y)$ respectively. We say that the random variables $X$ and $Y$ are statistically independent, if and only if $$f(x,y) = g(x)h(y)$$

The Bernoulli Process

More information: Wikipedia

Bernoulli was a 18th century Swiss science-guy after whom the Bernoulli Process is named. Bernoulli's encounters with women were the first Bernoulli trials, and he'd divide them into two categories: "I totally hit that" and "haven't hit it yet". Today, it is more common to use a more PC example when introducing students to the Bernoulli Process, such as a coin flip.

Experiments often consist of repeated trials, each with two possible outcomes that could be labeled success or failure; $1$ or $0$; heads or tails, and each of which are independent. This is called a Bernoulli process. Each trial (e.g. coin flip, drawing a ball at random and then putting it back) is called a Bernoulli trial. The Bernoulli process is a finite or infinite sequence of binary random variables. A common example of a Bernoulli process is repeated coin flipping (possibly with an unfair coin, but consistent unfairness). Drawing from a deck of cards is a Bernoulli process if the outcome is either success or failure (e.g. we could be interested in drawing a spade) and the cards are replaced between each selection, so that the probability of drawing a given card remains the same between draws.

Strictly speaking, a Bernoulli process is a finite or infinite sequence of independent random variables $X_{1}$, $X_{2}$, $X_{3}$, $\ldots$, with the following properties:

1. For each $i$, the value of $X_{i}$ is either $0$ or $1$.
   • (The experiment consists of repeated trials with the outcome $X_{i}$ which is either $0$ or $1$, i.e. the outcome is binary. We are free to redefine $1$ as "success" and $0$ as "failure" or $1$ as "vagina" and $0$ as "cats".)
2. For all values of $i$, the probability that $X_{i} = 1$ is the same number $p$.
   • (The probability of either of the two possible outcomes remain constant from trial to trial: this implies that the trials are independent.)

Binomial Distribution | Binomialfordeling | $b(x;n,p)$

A binomial distribution is the probability distribution of the discrete random variable $X$: the number of successes in $n$ Bernoulli trials. The values of a binomial distribution is denoted by $b(x;n,p)$, where $x$ is the value of the random variable $X$; $n$ is the number of trials and $p$ is the probability of the outcome being $1$/"success".

The probability distribution of the binomial random variable $X$ (the number of successes/$1$s in $n$ independent trials) in a Bernoulli trial with probability $p$ of success/$1$, is $$\begin{array}{cc} b(x;n,p) = \binom{n}{x}p^{x}(1-p)^{n-x}\text{,} && x = 0,1,2,\ldots,n.\end{array}$$

Note that $\sum_{x=0}^{n}b(x;n,p) = 1$; the sum of the probability of $0$, $1$, $2$, ..., $n$ successes occuring is $1$; the number of successes in a binomial distribution is an integer between or equal to $0$ and $n$.

Multinomial distribution | multinomisk fordeling

If we allow the trials of a binomial experiment to have more than two possible outcomes, it becomes a multinomial experiment.

If a given trial can result in one of the $k$ possible outcomes $E_{1}$, $E_{2}$, ..., $E_{k}$ with probabilities $p_{1}$, $p_{2}$, ..., $p_{k}$, then the multinomial distribution returns the probability that $E_{1}$, $E_{2}$, ..., $E_{k}$ occurs $x_{1}$, $x_{2}$, ..., $x_{k}$ times in $n$ independent trials: $$f(x_1,x_2,\ldots,x_k;p_1,p_2,\ldots,p_k,n) = \binom{n}{x_1,x_2,\ldots,x_k}p_{1}^{x_{1}}p_{2}^{x_{2}}\cdots p_{k}^{x_{k}}$$ Assuming $$\begin{array}{ccc} \sum_{i=1}^{k}x_{i} = n && \text{and} && \sum_{i=1}^{k}p_{i} = 1 \end{array}$$

Hypergeometric Distribution

Hypergeometric distributions are kind of like binomial ones except that they don't require independence between trials, which means that we can do stuff like draw from a deck of cards without replacing the card (and shuffling the deck) between each draw.

A hypergeometric distribution is the probability distribution of a hypergeometric random variable, $X$, classified as the number of successes of a hypergeometric experiment -- an experiment with the following properties:

1. A random sample of size $n$ is selected without replacement from $N$ items
2. Of the $N$ items, $k$ may be classified as successes and $N-k$ are classified as failures. As previously stated, the hypergeometric random variable $X$ is the number of successes in such an experiment.

The values of a hypergeometric distribution are denoted by $h(x;N,n,k)$ $$\begin{array}h(x;N,n,k) = \frac{\binom{k}{x}\binom{N-k}{n-x}}{\binom{N}{n}}\text{,} && \text{max}\{0,n-(N-k)\}\leq x\leq \text{min}\{n,k\}\end{array}$$

Negative binomial distribution | negativ-binomisk fordeling

The negative binomial distribution is the probability distribution of the number of successes in a sequence of Bernoulli trials before a specified number of failures occurs. Or the number of failures before a specified number of successes occur.

For example, we could throw a six sided die until we get a five for the third time. The probability distribution of the number of non-fives that we got will be negative binomial.

If repeated independent trials can result in success with probability $p$ and a failure with probability $1-p$, then the probability distribution of the random variable $X$ (the number of the trial on which the $k$th success occurs) is $$P(X = x) = \begin{array}b^{*}(x;k,p) = \binom{x-1}{k-1}p^{k}(1-p)^{x-k}\text{,} && x=k, k+1, k+2, \ldots.\end{array}$$

Geometric distribution | Geometrisk fordeling

The geometric distribution is a special case of the negative binomial distribution where $k=1$. That is, when the specified number of successes is $1$. $$b^{*}(x;1,p) = \binom{x-1}{1-1}p^{1}(1-p)^{x-1} = p(1-p)^{x-1}$$

The values of the geometric distribution is denoted by $g(x;p)$ $$\begin{array}g(x;p) = p(1-p)^{x-1}\text{,} && x = 1, 2, 3, \ldots.\end{array}$$

Example: We are flipping a balanced coin. What is the probability that we don't get heads until the fifth flip? $g(5;0.5) = 0.5(1-0.5)^{5-1} = 0.5(1-0.5)^{4}$

Poisson Distribution

The Poisson distribution expresses the probability of a given number of events occuring in a fixed interval of time and/or space if these events occur with a known average rate and independently of the time since the last event.

Given only the average rate of some event occuring for a certain period of observation (for example that Bob gets punched four times a day) and assuming that the process that produces the event is essentially random, the Poisson distribution specifies how likely it is that Bob will get punched 2, or 5, or 10, or any other number, during one period of observation.

The probability distribution of the Poisson random variable $X$, representing the number of outcomes occuring in a given time interval or specified region denoted by $t$, is $$\begin{array}p(x;\lambda t) = \frac{e^{-\lambda t}(\lambda t)^x}{x!}\text{,} && x = 0, 1, 2, \ldots.\end{array}$$ Where $\lambda$ is the average number of outcomes per unit time/distance/area or volume. In other words, $\lambda t$ is the expected value of $X$ for the time period $t$. So an easier way to think about it is $$p(x;E(X)) = \frac{e^{-E(X)}(E(X))^x}{x!}$$

Note that the little blue book uses $\mu$ as $\lambda t$, and that you are typically given $\mu$, $E(X)$ or $Var(X)$ in Poisson distribution related assignments.

Continuous Uniform Distribution | Kontinuerlig uniformfordeling

Normal Distribution | Gauss distribution | Normalfordeling | Gaussfordeling

The normal distribution is super double important.

The density of a normal random variable $X$, with mean $\mu$ and variance $\sigma^{2}$, is $$\begin{array} n(x;\mu,\sigma) = \frac{1}{\sigma \sqrt{2\pi}}e^{-\frac{1}{2\sigma^{2}}(x-\mu)^{2}}\text{,} && -\infty < x < \infty \end{array}$$ Did you notice how the density of $X$ is defined solely by its expected value and variance?

Did you also notice how the table of values for the normal distribution in the little blue book only covers $\mu = 0$ and $\sigma^{2} = 1$? Good on you. Thankfully we don't have to compute the integral of $n(x;\mu,\sigma)$ to find the probability of $X$ assuming some value. Although it is doable the result contains the error function and I don't even know what course that's covered in.

Tasked with finding the probability that the normal random variable $X$ assumes some value less than (or equal to, remember how the probability of a continuous random variable to assume any single value is $0$?) $a$ -- i.e. $P(X < a)$, with a given variance $\sigma^{2}$ and mean $\mu$, we calculate $$z_{a} = \frac{a - \mu}{\sigma}$$ and find $P(Z < z)$ instead, where $Z$ is a normal random variable with mean $\mu = 0$ and variance $\sigma^{2} = 1$. This means we can use the tables!

If tasked to find $P(a < X < b)$, calculate $z_{a}$ and $z_{b}$ and evaluate $P(z_{a} < X < z_{b})$.

Note: This "formula" is in the little blue book on page 31 (the map of relations between probability distributions), above the arrow going from Normal($\mu$, $\sigma^{2}$) to Normal($0,1$).

Gamma Distribution | Gammafordeling | $\Gamma$

The continuous random variable $X$ has a gamma distribution with parameters $a$ and $b$ if its density function is given by $$f(x;a,b) = \begin{array}{ll}\frac{1}{b^{a}\Gamma(a)}x^{a - 1}e^{\frac{-x}{b}} \text{,} && x > 0\text{,} \\ 0\text{,} && \text{elsewhere,}\end{array}$$ where $a > 0$ and $b > 0$.

The mean and variance of the gamma distribution are $$E(X) = \mu = a \beta$$ $$Var(X) = \sigma^{2} = a \beta^{2}$$

(Note that $\alpha$ and $\beta$ are used instead of $a$ and $b$ in both the book and the little blue book, but who's got time for that jazz. People still know what's up.)

The gamma function is defined by $$\begin{array}\Gamma(a) = \int_{0}^{\infty}x^{a-1}e^{-x}dx, && \text{for } a > 0\end{array}$$

Properties of the gamma function $$\Gamma(a) = (a-1)\Gamma(a-1)$$ $$\begin{array}{ll} \Gamma(n) = (n-1)! && \text{if } n \in \mathbb{Z}\end{array}$$ $$\Gamma(1) = 1$$ $$\Gamma(\frac{1}{2}) = \sqrt(\pi)$$

So yeah, the Gamma function is pretty dope and you better hope we don't have to do any calculations with it by hand.

Exponential distribution | Eksponensialdistribusjon

The exponential distribution is a special case of the gamma distribution where $a = 1$.

The continuous random variable $X$ has an exponential distribution, with parameter $b$, if its density function is given by $$f(x;b) = \begin{array} \frac{1}{b} e^{-\frac{x}{b}}, && x > 0, \\ 0, && \text{elsewhere}\end{array}$$ where $b > 0$

The mean and variance of the exponential distribution are $$E(X) = \mu = b$$ $$Var(X) = \sigma^{2} = b^{2}$$

The exponential function is pretty closely related to the Poisson process. This is apparently super useful. Check out page 196.

Chi-Squared Distribution | Kjikvadratfordeling | $\chi^{2}$-distribution

The Chi-Squared distribution is another special case of the gamma distribution where $a = \frac{v}{2}$ and $b=2$ ($v$ is a positive integer). The Chi-squared distribution has a single parameter, $v$, called the degrees of freedom.

The continuous random variable $X$ has a chi-squared distribution with $v$ degrees of freedom if its density function is given by $$f(x;v) = \begin{array} \frac{1}/{2^{v/2}\Gamma(v/2)}x^{(v/2)-1}e^{-(x/2)}, && x > 0, \\ 0, && \text{elsewhere} \end{array}$$ ($v$ is a positive integer).

The mean and variance are $$E(X) = \mu = v$$ $$Var(X) = \sigma^{2} = 2v$$

Transformation of Variables

Suppose that $X$ is a discrete random variable with probability distribution $f(x)$. Let $Y = u(X)$ define a one-to-one transformation between the values of $X$ and $Y$ so that the equation $y = u(x)$ can be uniquely solved for $x$ in terms of $y$, say $x = w(y)$. Then the probability distribution of $Y$ is $g(y) = f[w(y)]$

What does this mean?

If we have a discrete random variable $X$ with a known probability distribution function, and another function that defines a one-to-one mapping between $X$ and $Y$ (another discrete random variables with the exact same values as $X$), then the probability function of $Y$ is equal to the probability function of $x$ with the one-to-one mapping function between $Y$ and $X$ (the one that translates from $Y$ to $X$) as its argument.

Suppose that $X_{1}$ and $X_{2}$ are discrete random variables with joint probability distribution $f(x_{1}, x_{2})$. Let $Y_{1} = u_{1}(X_{1}, X_{2})$ and $Y_{2} = u_{2}(X_{1}, X_{2})$ def