1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Statistics - Poisson distribution question.

  1. Apr 29, 2013 #1
    1. The problem statement, all variables and given/known data
    I am somewhat perplexed by the proposed solution to the following Statistics problem and was hoping one of you might be willing to help me settle this:
    An operator receives phone calls between 8AM and 4PM at an average rate of 20 calls/hour. No call was received during the first 3 minutes. What is the probability that we shall have to wait at least one more minute before the first call is received?

    2. Relevant equations

    3. The attempt at a solution
    Now, the book states it ought to be P(T≥1) = 1 - FT(1) = e-1/3.
    My question is, why should it not have been 1 - [F(4) - F(3)], i.e. the complement of a call received between the 3rd and 4th minutes?
    I'd appreciate it if any of you could explain this to me.
  2. jcsd
  3. Apr 29, 2013 #2

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    If T is the time of the first arrival, you want P{T > 4 | T > 3}, because you are told that the event {T > 3} occurred.
  4. Apr 29, 2013 #3
    Thank you, Ray! It's clearer now :-).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted