Question

On discovering that her family had a 70% risk of heart attack, Erin took a treadmill test to check her own potential of having a heart attack. The doctors told her that the reliability of the stress test is 67%. The test predicted that Erin will not have a heart attack. What is the probability after the test was taken that she will not have a heart attack?

0.099

0.201

0.231

0.469

Answer 1

@Michele_Laino @ganeshie8

Answer 2

hint:
we have to multiply the single probabilities. Namely
the probability to have not heart attack is p_1=1-0.7=...?
the probability to have not heart attack from the test, is p_2=1-0.67=...?
so the requested probability, is:
p_1*p_2=...?

Answer 3

.3*.33?

Answer 4

.099 as my answer? :)

Answer 5

sorry, please:
p_2 =0.67

Answer 6

.201 as my answer

Answer 7

yes! that's right!

Answer 8

Let's see

Answer 9

it was wrong :(

Answer 10

try this

(0.3*0.67)/(0.3*0.67+0.7*0.33)

Answer 11

That gets .465

Answer 12

yes try last option

Answer 13

Alright I will next try

Answer 14

btw that expression comes from conditional probability

Answer 15

P("no heart attack" | "test giving no heart attack") = P("no heart attack" AND "test giving no heart attack")/P("test giving no heart attack")

Answer 16

there is a p_2= 33% of probability to have no heart attack, and we have p_1=30% to have not heart attack. So try with:
p_1*p_2=0.33*0.30=0.099

Answer 17

how can the probability of not having heart attack reduce from 30% to 9.9% even after the test showed no heart attack ?

Answer 18

actually we can eyeball the options without doing any work.

the probability after the test has to be greater than 30% whatever that is.
from your options only one is greater than 30% so..

Answer 19

I think that the test tell us that there is a 67% to have an heat attack, namely there is a 67% to confirm that 70% of probability to have an heat attack, so there is a 33% to have no heart attack from that test.

Answer 20

It is not .469

Answer 21

oh

Answer 22

I might be wrong, but i don't really see how the probability of "not having heart attack" reduces even after the test predicts "no heart attack"

Answer 23

Not sure

Answer 24

Well it's not B, C, D so it must've been your first answer 0.099 :\ because I got it wrong thinking it was .231

Answer 25

ITS NOT A!!!!

Answer 26

NOT B