In: Statistics and Probability
Assume that 40% of graduates come from University A and and earn salaries of 30,000 on average with a standard deviation of 5,000. The remainder come from University B and earn salaries of 25,000 on average with a standard deviation of 7,500. If you are told that a graduate is earning less than 35,000, what is the probability that they came from University A? (Assume salaries are normally distributed.)
P(z<Z) table :
P(universitiy A) = 0.40
P(universitiy B) = 0.60
P(salary <= 35000 | universitiy A) :
P(salary <= 35000 | universitiy A) = 0.8413
P(salary <= 35000 | universitiy B) :
P(salary <= 35000 | universitiy B) = 0.9088
P(salary <= 35000) = P(salary <= 35000 | universitiy A)*P(universitiy A) + P(salary <= 35000 | universitiy B)*P(universitiy B)
= 0.8413*0.40 + 0.9088*0.60
= 0.8818
P(university A | salary <= 35000) = P(salary <= 35000 | universitiy A)*P(universitiy A) / P(salary <= 35000)
= 0.8413*0.40 / 0.8818
= 0.3816
P(university A | salary <= 35000) = 0.3816
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