M4.6.7 For the adult
Conditional probability
In a class of 60, some pupils walk to school and some take the bus. If I pick a girl, what is the chance that she walks?
What this chapter teaches
A conditional probability is the chance of one event when another is known to have happened. In a two way table, look only at the row or column of the known event. The rule is P(B given A) = P(A and B) ÷ P(A). P(A given B) and P(B given A) are usually different.
Three questions to ask
There is no one right wording. Listen for the idea.
- In the walk and bus table, why is the chance for a girl found by dividing by 30 and not by 60?
- Why are P(walks given girl) and P(girl given walks) not the same?
- How can you tell from the numbers that two events are independent?
After the chapter, the learner can
- Find a probability given that something has already happened
- Use a two way table to find a conditional probability
What they copy into their notebook
P(B given A): look only at the row or column of A. P(B given A) = P(A and B) ÷ P(A). P(B given A) and P(A given B) differ. Independent: P(B given A) = P(B).