Originally Posted by schoolboy46
Personally, have always had trouble with Bayes Theory! Here is how I would do this:
Imagine a hospital with 10000 patients (chosen to give integer numbers). "From hospital records, the researchers know that 23% of all hospital patients are smokers, while 77% are nonsmokers." Okay, so 2300 patients are smokers, 7700 are non-smokers. "Researchers have determined that patients who are smokers have an 18% chance of contracting a serious illness" So .18*2300= 414 so 414 of the smokers have a serious illness. "there is only a .06 probability that a nonsmoker will contract a serious illness." .06*7700= 462 so 462 of the non-smokers have a serious illness.
To find "the probability that a given patient is a smoker if the patient has a serious illness", note that there are a total of 414+ 462= 876 patients with a serious illness, of whom 414 are smokers.
Problem # 12):
The Senate consists of 100 senators, of whom 34 are Republicans and 66 are Democrats. A bill to increase defense appropriations is before the Senate. Thirty- five percent of the Democrats and 70% of the Republicans favor the bill. The bill needs a simple majority to pass. Using a probability tree, determine the probability that the bill will pass.
I think the formula that I want to use here is P(A or B)= P(A) + P (B) - P(AB), but don't know how to take it from here as far as actually setting up a tree...
A= 66 Democrats
B= 34 Republicans
P (B)= 0.70
P(A or B)= 66*0.35 + 34*0.70-(.35*.7)= not adding up??
Problem # 14):
A metropolitan school system consists of three districtsó north, south, and central. The north dis-trict contains 25% of all students, the south district contains 40%, and the central district contains 35%. A minimum- competency test was given to all students; 10% of the north district students failed, 15% of the south district students failed, and 5% of the central district students failed.
a. Develop a probability tree showing all marginal, conditional, and joint probabilities.
b. Develop a joint probability table.
c. What is the probability that a student selected at random failed the test?
Don't have a clue where to start based on the one above?