ProbabilityProbability

By Laura Farrington 8GTBy Laura Farrington 8GT

What is Probability?What is Probability?

Probability is about the chance of something happening. When we talk about how probable something is we say things like 'unlikely', 'probably', 'even chance', 'almost definitely' and '100% certain'. All these are ways of talking about probability.How can we use numbers to describe probabilities? The first step is the probability scale.

                                                                                  

Probability ScaleProbability Scale

The chances of any event can be shown on a probability scale from 0 to 1.•A probability of 0 tells us that the event will never happen - it's impossible •A probability of 1 tells us that the event is certain to happen •A probability of 1/2 tells us that the event has an even chance of happening

                                                                               

Probability TreesProbability Trees

A bag contains 4 red and 6 blue sweets. One sweet is

taken out and then put back. A second sweet is then

removed. Here is a tree diagram showing the possible

outcomes.

A probability tree shows all the possible outcomes of an event. Here is an example of a probability tree:

Throwing a CoinThrowing a Coin There is a one in two chance of getting heads when a coin is tossed. This can be written as a fraction. It's a probability of ½. When a coin is tossed you can get either a head or a tail. So there are 2 possible outcomes.But if we want heads then only 1 outcome is successful. So we want 1 outcome out of the 2 possible outcomes. We can write this as a fraction, 1/2.

                                                                                   

Rolling a diceRolling a dice

There are six possible outcomes                                                     Only one outcome is successful, the 3. We make a fraction to work it out.

                                                                           

The probability of any of the numbers on a dice is 1/6. They are all equally likely.