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Homework Help: Normal distribution problem

  1. Mar 13, 2007 #1
    1. The problem statement, all variables and given/known data
    Plastic rods are cut into nominal length of 6 inches. Actual lengths are normally distributed about a mean of 6 inches and their standard deviation is 0.06 inch.

    Question: To what value does the standard deviation need to be reduced if 99% of the rods must be within tolerance?

    2. Relevant equations
    sd=standard deviation
    u=mean
    P(a<X<=b)=F((b-u)/(sd))-F((a-u)/(sd))

    3. The attempt at a solution
    since they want the possibility of rods to be between u+sd and u-sd to be 0.99, b=u+sd and a=u-sd
    and the equation will become
    P(a<X<=b)=F((u+sd-u)/(sd))-F((u-sd-u)/(sd))
    F(1)-F(-1) doesn't equal to 0.99.

    Am I misinterpreting the word tolerance?
    I dont know what else to try... please help thank you!
     
    Last edited: Mar 13, 2007
  2. jcsd
  3. Mar 13, 2007 #2
    nevermind i misread the problem...
     
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