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What is probability?
We use probability to measure how likely it is that something will happen. We can describe probability as, for example, a one-in-four chance, or write it as a decimal (0.25), a fraction (1/4), or as a percentage (25%). Probability is part of a branch of mathematics called statistics, which is about understanding data.
How likely is it?
If something is likely to happen, we say it has a high probability. If it is unlikely, we say it has a low probability.
If something is definitely going to happen, we say it is certain. For example, in a jar containing only yellow marbles, it is certain that when we pick one out, it will be yellow. In this case, the probability is 1, 10/10, or 100%.
Outcomes
In probability, when something happens, the result is called an outcome.
When you flip a coin, there are two possible outcomes: “heads” or “tails”.
When you roll a dice, there are six possible outcomes: rolling a 1, 2, 3, 4, 5 or a 6.
Working out probability
To work out the probability that an event will happen, we need to know two things: the number of ways it can happen, and the total number of outcomes possible.
For example, let’s say we want to work out the probability that when you reach into a bag of sweets, you will pick out a red one. There are 10 sweets in the bag altogether and 3 of them are red.
To work out the probability of picking a red sweet we use the formula:
Probability = number of ways something can happen / total number of outcomes
The number of ways to pick a red sweet = 3 (there are 3 red sweets which could be picked) and the total number of outcomes is 10 (there are 10 sweets in total which could be picked).
3 / 10 = 0.3
So the probability of picking a red sweet = 0.3
We can also write this as a fraction (3/10), or as a percentage (30%), or describe it as a three-in-ten chance.
Writing probability
The most common way of writing probability is as a fraction. The closer a fraction is to a whole (1), the higher the probability, and the more likely an event is to happen.
Probability can also be written as a percentage, which tells us the likelihood out of 100 that something will happen. A probability of 100% means that something is certain to happen. 50% probability means that it is equally likely to happen or not happen. 0% probability means that it definitely won't happen.
When probability is written as a decimal, it can only be a number between 0 and 1. The closer the number is to 1, the higher the probability. Something with a probability of 0.8 or 0.9 is very likely to happen, while something with a probability of 0.1 or 0.2 is unlikely to happen.
Consultant: Adam Larking
Key words
certain definitely going to happen
outcome the result, for example, pulling a red sweet from the bag, or flipping a coin with "heads" facing up
probability how likely it is that something will happen
statistics the study of data
pics
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