The following table gives the results of a survey in which infants in the survey are classified as having been immunized against some childhood disease, not immunized, whether the child's family has been classified as being below the poverty level, or not below the poverty level.
Below Poverty 
Not Below Poverty 

Immunized 
5 
55 
Not immunized 
15 
25 
What is the probability that an infant has not been immunized given
the family has been classified as being below the poverty
level?
Let , A be the event that an infant has not been immunized and B be the event that the family has been classified as being below the poverty level.
Now , be the event that the an infant has not been immunized and the family has been classified as being below the poverty level. Since , the total number of observations is 5+55+15+25=100
Also , the total number of observations in B and is , n(B)=5+15=20 and
Therefore , by using classical definition of probability ,
Therefore , by using definition of conditional probability ,
Therefore , the probability that an infant has not been immunized given the family has been classified as being below the poverty level is 0.75
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