- #1
mrmt
- 6
- 0
Hi guys,
I can't get my head around this, if anyone could help that would be great.
"A robotic assembly line contains 20 stations. Suppose that the probability
that each individual station will fail is 0.3 and that the stations fail indepen-
dently of each other. Given that at least one of the stations has failed, what is
the probability that at least 2 of the stations have failed?"
Binomial distribution as:
fixed no. of identical trials
only 2 outcomes (success/failure)
the trials are independent
Slightly confusing wording, but if we call a "success" failure of a station, what is:
P(X≥2 l X≥1) ?
First I played around with just a reduced sample space i.e. for X≥2 out of the original 20 stations give that X≥1 already, would mean that X≥1 out of the remaining 19? i.e. 1-P(X=0)
(For the above I assumed that although the wording says "at least one station" has already failed, that would have to mean "only one station has failed" as if any more that one station had failed already the probability that X≥2 would be 1 (a certainty).)
I've seen two other similar examples floating around the web, one with
P(X≥2)/P(X≥1)
and another I still can't understand: (figures changed to suit this problem, wording unchanged)
"failure has occurred it is no longer just a possibility
the binomial from 0 to n C(20,s).7^(20-s).3^s no longer applies s the number of failures
since failure is known to have happened we exclude .7^20
we are in that bleak place where success is impossible probability (1 - .7^20) which is parcelled out among the various possibilities of failure to sum to one. We scale these by dividing through by (1 - .7^20)
so the probability of two or more failures is (certain failure - (C(20,1).1^1*.9^9)/(1-.9^10)) = (certain failure - p(only one failure))
(1 - (C(20,1).3^1*.7^19)/(1-.7^20)"
Any help would be great
Cheers
I can't get my head around this, if anyone could help that would be great.
"A robotic assembly line contains 20 stations. Suppose that the probability
that each individual station will fail is 0.3 and that the stations fail indepen-
dently of each other. Given that at least one of the stations has failed, what is
the probability that at least 2 of the stations have failed?"
Binomial distribution as:
fixed no. of identical trials
only 2 outcomes (success/failure)
the trials are independent
Slightly confusing wording, but if we call a "success" failure of a station, what is:
P(X≥2 l X≥1) ?
First I played around with just a reduced sample space i.e. for X≥2 out of the original 20 stations give that X≥1 already, would mean that X≥1 out of the remaining 19? i.e. 1-P(X=0)
(For the above I assumed that although the wording says "at least one station" has already failed, that would have to mean "only one station has failed" as if any more that one station had failed already the probability that X≥2 would be 1 (a certainty).)
I've seen two other similar examples floating around the web, one with
P(X≥2)/P(X≥1)
and another I still can't understand: (figures changed to suit this problem, wording unchanged)
"failure has occurred it is no longer just a possibility
the binomial from 0 to n C(20,s).7^(20-s).3^s no longer applies s the number of failures
since failure is known to have happened we exclude .7^20
we are in that bleak place where success is impossible probability (1 - .7^20) which is parcelled out among the various possibilities of failure to sum to one. We scale these by dividing through by (1 - .7^20)
so the probability of two or more failures is (certain failure - (C(20,1).1^1*.9^9)/(1-.9^10)) = (certain failure - p(only one failure))
(1 - (C(20,1).3^1*.7^19)/(1-.7^20)"
Any help would be great
Cheers