Question

Allergic reactions to poison ivy can be miserable. Plant oils cause the reaction. Researchers at Allergy Institute did a study to determine the effects of washing the oil off within 5 minutes of exposure. A random sample of 1000 people with known allergies to poison ivy participated in the study. Oil from the poison ivy plant was rubbed on a patch of skin. For 500 of the subjects, it was washed off within 5 minutes. For the other 500 subjects, the oil was washed off after 5 minutes. The results are summarized in the following table.

Reaction	Within 5 Minutes	After 5 Minutes	Row Total
None Mild Strong	420 63 17	64 323 113	484 386 130
Column Total	500	500	1000

Let's use the following notation for the various events: W = washing oil off within 5 minutes, A = washing oil off after 5 minutes, N = no reaction, M = mild reaction, S = strong reaction. Find the following probabilities for a person selected at random from this sample of 1000 subjects. (Use 3 decimal places.)

(a)	P(N)	1
	P(M)	2
	P(S)	3

(b)	P(N | W)	4
	P(S | W)	5

(c)	P(N | A)	6
	P(S | A)	7

(d)	P(N and W)	8
	P(M and W)	9

(e) P(N or M).
10
Are the events N = no reaction and M = mild reaction mutually exclusive? Explain.

Yes. P(N and M) = 0. No. P(N and M) ≠ 0.     No. P(N or M) ≠ 0. Yes. P(N or M) = 0.

(f) Are the events N = no reaction and W = washing oil off within 5 minutes independent? Explain.

Yes. P(N and W) = P(N) · P(W). No. P(N and W) = P(N) · P(W).     No. P(N and W) ≠ P(N) · P(W). Yes. P(N and W) ≠ P(N) · P(W).

Answer 1