Barbara Illowsky; Susan Dean

Bernoulli trials

an experiment with the following characteristics:

There are only two possible outcomes called success and failure for each trial
The probability p of a success is the same for any trial (so the probability q = 1 − p of a failure is the same for any trial)

binomial experiment

a statistical experiment that satisfies the following three conditions:

There are a fixed number of trials, n
There are only two possible outcomes, called success and, failure, for each trial; the letter p denotes the probability of a success on one trial, and q denotes the probability of a failure on one trial
The n trials are independent and are repeated using identical conditions

binomial probability distribution: a discrete random variable (RV) that arises from Bernoulli trials; there are a fixed number, n, of independent trials
Independent means that the result of any trial (for example, trial one) does not affect the results of the following trials, and all trials are conducted under the same conditions. Under these circumstances the binomial RV X is defined as the number of successes in n trials. The notation is: X ~ B(n, p). The mean is μ = np and the standard deviation is σ = $\sqrt{n p q}$ . The probability of the following exactly x successes in n trials is
P(X = x) = $(\begin{array}{l} n \\ x \end{array})$ p^xq^{n − x}

expected value: expected arithmetic average when an experiment is repeated many times; also called the mean; notations μ; for a discrete random variable (RV) with probability distribution function P(x),the definition can also be written in the form μ = $\sum$ xP(x)

geometric distribution: a discrete random variable (RV) that arises from the Bernoulli trials; the trials are repeated until the first success.
The geometric variable X is defined as the number of trials until the first success. Notation X ~ G(p). The mean is μ = $\frac{1}{p}$ and the standard deviation is σ = $\sqrt{\frac{1}{p} (\frac{1}{p} - 1)}$ . The probability of exactly x failures before the first success is given by the formula
$P (X = x) = p {(1 - p)}^{x - 1}$
.

geometric experiment

a statistical experiment with the following properties:

There are one or more Bernoulli trials with all failures except the last one, which is a success
In theory, the number of trials could go on foreve; there must be at least one trial
The probability, p, of a success and the probability, q, of a failure do not change from trial to trial

hypergeometric experiment

a statistical experiment with the following properties:

You take samples from two groups
You are concerned with a group of interest, called the first group
You sample without replacement from the combined groups
Each pick is not independent, since sampling is without replacement
You are not dealing with Bernoulli trials

hypergeometric probability

a discrete random variable (RV) that is characterized by the following:

The experiment uses a fixed number of trials.
The probability of success is not the same from trial to trial

We sample from two groups of items when we are interested in only one group. X is defined as the number of successes out of the total number of items chosen. Notation X ~ H(r, b, n), where r = the number of items in the group of interest, b = the number of items in the group not of interest, and n = the number of items chosen.

mean: a number that measures the central tendency; a common name for mean is average
The term mean is a shortened form of arithmetic mean. By definition, the mean for a sample (denoted by $\bar{x}$ ) is $\bar{x} = \frac{Sum of all values in the sample}{Number of values in the sample}$ and the mean for a population (denoted by μ) is μ = $\frac{Sum of all values in the population}{Number of values in the population}$ .

mean of a probability distribution: the long-term average of many trials of a statistical experiment

Poisson probability distribution

a discrete random variable (RV) that counts the number of times a certain event will occur in a specific interval; characteristics of the variable:

The probability that the event occurs in a given interval is the same for all intervals
The events occur with a known mean and independently of the time since the last event

The distribution is defined by the mean μ of the event in the interval. Notation X ~ P(μ). The mean is μ = np. The standard deviation is

σ = \sqrt{μ}

. The probability of having exactly x successes in r trials is

P (X = x) = (e^{- μ}) \frac{μ^{x}}{x!}

. The Poisson distribution is often used to approximate the binomial distribution, when n is large and p is small (a general rule is that n should be greater than or equal to 20 and p should be less than or equal to .05).

probability distribution function (PDF): a mathematical description of a discrete random variable (RV), given either in the form of an equation (formula) or in the form of a table listing all the possible outcomes of an experiment and the probability associated with each outcome

random variable (RV)

a characteristic of interest in a population being studied; common notation for variables are uppercase Latin letters X, Y, Z, . . . ; common notation for a specific value from the domain (set of all possible values of a variable) are lowercase Latin letters x, y, and z
For example, if X is the number of children in a family, then x represents a specific integer 0, 1, 2, 3, . . . ; variables in statistics differ from variables in intermediate algebra in the two following ways:

The domain of the random variable (RV) is not necessarily a numerical set; the domain may be expressed in words; for example, if X = hair color then the domain is {black, blond, gray, green, orange}
We can tell what specific value x the random variable X takes only after performing the experiment

standard deviation of a probability distribution: a number that measures how far the outcomes of a statistical experiment are from the mean of the distribution

the law of large numbers: as the number of trials in a probability experiment increases, the difference between the theoretical probability of an event and the relative frequency probability approaches zero

Key Terms