1. Probability
OCR Stage 6

2. Probability
In an experiment, things that can happen are called possible outcomes
Each of these has a possibility

3. Probability of an event happening
Probability = Required Outcome Total Possible Outcomes
i.e. The number of required outcomes divided by the total number of possible outcomes

4. Getting a 4 on a single dice
There is 1 required outcome – a four
There are 6 possible outcomes
P(4) = 1
The answer may be written as a fraction, decimal or percentage 6
NEVER as
“one out of six”
The P says, “The probability of”

5. The probability of not getting a 4?
There are five numbers on the single dice that are not a 4
There are six possible outcomes
So, P(Not getting a 4) = 5 6

6. 1/6 + 5/6 = 1 The total probabilities of any event have a total of 1
It is important that you notice that the two probabilities add together to give 1
The total probabilities of any event have a total of 1
1/6 + 5/6 = 1
Probability of not getting a 4 on a single dice
Probability of getting a 4 on a single dice

7. Sum of all probabilities equal 1.
Example
The probability of a football team’s results in the next match are:
P(Win) = 0.3
P(Lose) = 0.5
What is the probability of a draw?

8. Remember the total of all the probabilities is 1
The team must win, lose or draw.
P(Win) + P(Lose) + P(Draw) = 1
P(Draw) = 1
0.8 + P(Draw) = 1
P(Draw) = 0.2
This is 0.5
This is 0.3

9. Mutually Exclusive events
Can NOT happen at the same time
Examples
A HEAD and a TAIL when coin is spun
A 6 and an odd number when die rolled
A level 5 and a level 6 on a SAT paper

10. Example Sam and Pete play chess P(Sam wins) = 0.4 P(Pete wins) = 0.3
What is P(Draw) ?
P(Draw) = 1
0.7 + P(Draw) = 1
P(Draw) = 0.3

11. Exercises – to be completed next lesson
OCR Stage 5/6 Text,
Stage 6, Chapter 4, Page 182
Ex 4.1A
Ex 4.1B
Ex 4.2A
Ex 4.2B