Probability » Conditional probabilities and (in)dependence  


If a table is given and a probability is asked, watch out, as may be you have a conditional probability.

With a conditional probability, the total number of possibilities is not the total number of outcomes. Instead you have a subgroup.

Example
Boys Girls Total
Left-handed 3 4 7
Right-handed 8 11 19
Total 11 15 26

Question 1

Calculate the possibility of a person being left-handed.

Answer:
The number of good outcomes is 7.
The total number of possible outcomes is 26.
The probability is therefore 726

Question 2

Calculate the probability a girl is a right-handed person.

Answer:
The number of good outcomes is 11.
The total number of possible outcomes is 15.
The probability is therefore 1115

Question 3

Calculate the possibility a right-handed person is a girl.

Answer:
The number of good outcomes is 11.
The total number of outcomes is 19.
The probability is therefore 1119

As you can see, it is important to read the question carefully.