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: Simple statistics question.

  1. Jan 30, 2014 #1
    1. The problem statement, all variables and given/known data

    This was an example but I am not sure why there is a difference in the way they are calculating the standard deviation.

    A type of battery is produced with an average voltage of 60 with a standard deviation of 4 volts.

    A: If 9 batteries are selected at random, what is the probability that the total voltage of the 9 batteries is greater than 530?
    B: What is the probability that the average voltage of the 9 batteries is less than 62?

    2. Relevant equations

    3. The attempt at a solution

    Problem A: For the stdev they just: sqrt( 9 * 4^2) = 12

    Problem B: For the stdev they just: 4/sqrt(9) = 1.3333

    I get why Problem B is the way it is. We are dealing with a sample and we have the population stdev and we need to approximate the sample standard deviation by dividing by sqrt(n). But for Problem A, I feel like I am missing something simple. Any clarification would be great.
  2. jcsd
  3. Jan 30, 2014 #2

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    The total and the average have different variances. Go back and review your formulas.
  4. Jan 30, 2014 #3
    V[9X] = 81 * V[X] = 81 * (1.33333) = 144 and then the stdev = 12

    Sorry, for some reason I had never put that together.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted