Åk 6–9
 
English
2.1 What is Meant by Probability?
Probability is the chance or risk that different events occur.  Probability can be calculated for one or more events.  An event in probability has a number of possible outcomes. Outcomes are the possible results of an event.

Example A
What is the probability of rolling a 6 on one die? This example is a calculation of probability for one event.

Example B
What is the probability of rolling three 6's in a row? This example is a calculation of probability where several events can occur.


Foto: Fredrik Enander


For different events to be defined with probability means that you can mathematically calculate the chance or risk that they occur.

Probability usually is given with a number between 0 and 1 where:

  • 0 means that the event cannot occur.
  • 1 means that it will occur with certainty.

The closer to 0 the number comes, the less probable that the event that you are investigating will occur.

The closer to 1 the number comes, the more probable that the event that you are investigating will occur.




Uniform probability distributions
Throwing dice is a good example of independent probability distributions. Throwing dice is based on chance where the probability is equal for every one of the events among all possible events.

The possible events when throwing a dice are 1, 2, 3, 4, 5 and 6 and all of these events have the same probability.  This means that the probability distribution is uniform.

Probability can be calculated using its definition.






Mathematical Words

Experiment

An experiment with at least two possible outcomes where we don’t know ahead of time what the results will be.  Pulling out a card from a deck is an example of an experiment.


Outcome The results we get when we do an experiment is called an outcome. The card we pull out of the deck is an outcome. 

Event

An event consists of one or more outcomes.  If we for example, pull out a card from a deck, the event “pulling out a heart” consists of 13 events because there are 13 hearts in the deck.


Favorable Outcome

All those outcomes which match the criteria.  If you for example, pull out a heart from a deck, the number of favorable outcomes is 13, in other words all the hearts.


Possible Outcomes

Possible outcomes describes, just as it sounds, all the possible outcomes in an experiment. When you pull out a card from a deck there are 52 possible outcomes because the deck contains 52 cards.