What is a discrete probability distribution?
A Discrete Probability Distribution tells you the various probabilities associated with a discrete random variable. What does this mean? The definition of the word `distribution’ refers to how something is shared out in a group or how it is spread out over an area. Let X be a random variable – a variable whose possible values are the outcomes of a random event (see more on Random Variables). If X is a discrete random variable then its possible values belong to a discrete set of outcomes. If there are three possible outcomes then:
For example, a spinner is numbered 1, 2 and 3 with probabilities of 0.2, 0.35 and 0.45 respectively. This is an example of a discrete random variable and its associated discrete probability distribution.
The Uniform Distribution
The uniform probability distribution describes a discrete distribution where each outcome has an equal probability. Consider a random variable X that has a discrete uniform distribution. X can take one of k values:
If all these values all equally likely then they must each have a probability of 1/k. This is because they must all add up to 1.
The Binomial Distribution
The binomial probability distribution is closely related to Binomial Expansion. It gives the probability distribution of a random variable X that is subject to a number of trials. Each trial is independent (probabilities do not change from trial to trial) has a probability of success or failure. For example, the number of times a head comes up when a coin is tossed repeatedly is a binomial random variable. Each coin toss is a trial and probability of success is the probability of tossing heads.
In general, suppose there are n trials and each trial has a probability of success p. We write where means ‘distribution’ and ‘B’ means Binomial. The probability of having r successes is and given by:
Look up Binomial Expansion to calculate the ‘n choose r’ term in brackets. Now see Example 1 below.
We can use the tables in the Edexcel Formula Booklet (pages 29-33) to find cumulative probabilities. This is where you are looking for the probability of r successes or fewer: . Look for your value of n down the left hand side of each box in the table. Look for your value of p across the top. You will then see the r (or x) values on the left of each box for which you can read off the corresponding probability inside the table. See Example 2 below.
See Conducting Hypothesis Tests to see Binomial Distributions being used in Hypothesis Testing.