# Methods and formulas for Probability Density Function (PDF)

Select the method or formula of your choice.

## Probability density function

The probability density function (PDF) of a random variable, X, allows you to calculate the probability of an event, as follows:
• For continuous distributions, the probability that X has values in an interval (a, b) is precisely the area under its PDF in the interval (a, b).
• For discrete distributions, the probability that X has values in an interval (a, b) is exactly the sum of the PDF (also called the probability mass function) of the possible discrete values of X in (a, b).
Use PDF to determine the value of the probability density function at a known value x of the random variable X.

## Binomial distribution

The binomial distribution is used to represent the number of events that occurs within n independent trials. Possible values are integers from zero to n.

### Formula

mean = np

variance = np(1 – p)

The probability mass function (PMF) is:

Where equals .

In general, you can calculate k! as

### Notation

TermDescription
n number of trials
x number of events
p event probability

## Chi-square distribution

If X has a standard normal distribution, X2 has a chi-square distribution with one degree of freedom, allowing it to be a commonly used sampling distribution.

The sum of n independent X2 variables (where X has a standard normal distribution) has a chi-square distribution with n degrees of freedom. The shape of the chi-square distribution depends on the number of degrees of freedom.

### Formula

The probability density function (PDF) is:

mean = v

variance = 2v

### Notation

TermDescription
ν degrees of freedom
Γgamma function
e base of the natural logarithm

## Discrete distribution

A discrete distribution is one that you define yourself. For example, suppose you are interested in a distribution made up of three values −1, 0, 1, with probabilities of 0.2, 0.5, and 0.3, respectively. If you enter the values into columns of a worksheet, then you can use these columns to generate random data or to calculate probabilities.

Value Prob
−1 0.2
0 0.5
1 0.3

## Exponential distribution

The exponential distribution can be used to model time between failures, such as when units have a constant, instantaneous rate of failure (hazard function). The exponential distribution is a special case of the Weibull distribution and the gamma distribution.

### Formula

The probability density function (PDF) is:

The cumulative distribution function (CDF) is:

mean = θ + λ

variance = θ2

### Notation

TermDescription
θ scale parameter
λ threshold parameter
exp base of the natural logarithm
###### Note

Some references use 1 / θ for a parameter.

## F-distribution

The F-distribution is also known as the variance-ratio distribution and has two types of degrees of freedom: numerator degrees of freedom and denominator degrees of freedom. It is the distribution of the ratio of two independent random variables with chi-square distributions, each divided by its degrees of freedom.

### Formula

The probability density function (PDF) is:

### Notation

TermDescription
Γgamma function
u numerator degrees of freedom
v denominator degrees of freedom

## Geometric distribution

The discrete geometric distribution applies to a sequence of independent Bernoulli experiments with an event of interest that has probability p.

### Formula

If the random variable X is the total number of trials necessary to produce one event with probability p, then the probability mass function (PMF) of X is given by:

and X exhibits the following properties:

If the random variable Y is the number of nonevents that occur before the first event (with probability p) is observed, then the probability mass function (PMF) of Y is given by:

and Y exhibits the following properties:

### Notation

TermDescription
X number of trials to produce one event, Y + 1
Y number of nonevents that occur before the first event
p probability that an event occurs on each trial

## Integer distribution

The integer distribution is a discrete uniform distribution on a set of integers. Each integer has equal probability of occurring.

## Lognormal distribution

A variable x has a lognormal distribution if log(xλ ) has a normal distribution.

### Formula

The probability density function (PDF) is:

The cumulative distribution function (CDF) is:

### Notation

TermDescription
μ location parameter
σ scale parameter
λ threshold parameter
π Pi (~3.142)

## Normal distribution

The normal distribution (also called Gaussian distribution) is the most used statistical distribution because of the many physical, biological, and social processes that it can model.

### Formula

The probability density function (PDF) is:

The cumulative distribution function (CDF) is:

mean = μ

variance = σ 2

standard deviation = σ

### Notation

TermDescription
expbase of the natural logarithm
π Pi (~3.142)

## Poisson distribution

The Poisson distribution is a discrete distribution that models the number of events based on a constant rate of occurrence. The Poisson distribution can be used as an approximation to the binomial when the number of independent trials is large and the probability of success is small.

### Formula

The probability mass function (PMF) is:

mean = λ

variance = λ

### Notation

TermDescription
e base of the natural logarithm

## t-distribution

The t-distribution converges to the normal distribution as the degrees of freedom increase. The t-distribution is useful to do the following:
• Creating confidence intervals of the population mean from a normal distribution when the variance is unknown.
• Determining whether two sample means from normal populations with unknown but equal variances are significantly different.
• Testing the significance of regression coefficients.

### Formula

The probability density function (PDF) is:

mean = 0, when ν > 0

### Notation

TermDescription
Γgamma function
v degrees of freedom
π Pi (~3.142)

## Uniform distribution

The uniform distribution characterizes data over an interval uniformly, with a as the smallest value and b as the largest value.

### Formula

The probability density function (PDF) is:

TermDescription
a lower endpoint
b upper endpoint

## Weibull distribution

The Weibull distribution is useful to model product failure times.

### Formula

The probability density function (PDF) is:

The cumulative distribution function (CDF) is:

### Notation

TermDescription
α scale parameter
β shape parameter, when β = 1 the Weibull PDF is the same as the exponential PDF
λ threshold parameter
Γ gamma function
exp base of the natural logarithm
###### Note

Some references use 1/α as a parameter.

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