Probability…What is it? Probability is a value ranging from 0 to 1 that expresses the likelihood of an event happening. P(event) = successful outcomes Possible outcomes Can be expressed as a fraction, a percent or a decimal.

Certain event Has a probability of 1. Ex: What is the probability that you will come to school today?

Impossible event An event that has a probability of zero. Ex. What is the probability that you will not come to school today

Likely event An event that has a probability greater than ½. Example: On a die, what is the probability of getting a number > 1?

Unlikely event An event with a probability less than ½. Ex: On a die, the chance of getting a number less than 2.

Equally likely events Events that have an equal chance of happening. In a deck of cards, the probability of picking the King of spades of the ace of hearts.

Experiment An activity in which results are observed.

trial Each observation in an experiment

Outcome Each result in an experiment

Sample Space The list of all possible outcomes. For example, if you toss a coin, the possible outcomes are: Heads (H) or Tails (T)

Event Any set of one or more outcomes

Complementary probabilities Probabilities which represent the whole sample space. Example…tossing a coin P(H) and P(T) = entire sample space These probabilities add to 1.