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: Z proportions

  1. Sep 6, 2011 #1
    1. The problem statement, all variables and given/known data

    Non-Germinated ---405-----341
    Germinated --------386-----180

    Carry out the Z-test to compare the proportions of seeds germinating for the IG and IB columns and interpret the result.

    2. Relevant equations

    3. The attempt at a solution

    I don't know what I'm doing wrong but I'm assuming it's something because my answer is too big for the Z-table

    Z = (p1-p2)/root([(p1(1-p1)/n1)+[(p2(1-p2)/n2)

    P1 = 386/791
    P2 = 341/521

    Then throw everything into the Z formula. What's wrong with this?
  2. jcsd
  3. Sep 6, 2011 #2
    Any advice?
  4. Sep 6, 2011 #3

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    Well, for one thing, you should use p1 = 386/791 and p2 = 180/521, because you want to compare similar proportions for columns 1B and 1G.

    For z-values beyond the table you can use a simple asymptotic approximation: for Z a standard normal random variable we have Pr{Z > z} ~ exp(-z^2/2)/[z*sqrt(2*Pi)] for large z. For example, for z = 3 the exact value (to two decimal places) is Pr(Z > 3} = 1.35e-3 while the approximation is 1.48e-2, while for z = 4 the exact value is 3.17e-5 and the approximation is 3.35e-5. There are improved simple approximations available. For example, the approximation P{Z . z} ~ (1/z - 1/z^3)*exp(-z^2/2)/sqrt(2*Pi) gives much better accuracy.

  5. Sep 6, 2011 #4
    Ah, I see the mistake.

    Our professor says we answers shouldn't go beyond the table, so is my formula incorrect, should I use the formula from here instead?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook