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Probability & Central Limit Theorem

 
 
rihnavy
 
Reply Wed 19 Oct, 2016 04:30 pm
The population mean and standard deviation are given below. Find the required probability and determine whether the given sample mean would be considered unusual.
μx̄ = μ = 12,749
σ = 1.2
n = 35

For the given sample n = 35, the probability of a sample mean being less than 12,749 or greater than 12,752 is _________
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rihnavy
 
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Reply Thu 20 Oct, 2016 05:23 pm
@rihnavy,
This is what I have so far.
σx = σ/ √n = 1.2/ √35 = 0.2028

z = x̄ - μx̄/σx = x̄ - μx̄/σ/√n = 12,749 - 12,752/ 1.2√35 = -0.4226 = .33724

I stopped right there because I got confused. I'm stuck.
engineer
 
  1  
Reply Thu 20 Oct, 2016 06:58 pm
@rihnavy,
If I understand your problem correctly, you have your formula slightly wrong.

z = (x̄ - μx̄)/σx =( x̄ - μx̄)/(σ/√n) = (12,749 - 12,752)/ (1.2√35) = -14.8

If you look up this z score in a table or just use an internet calculator, you see that the chance of being fourteen standard deviations from the mean is zero. Since that is a really unusual answer, it makes me wonder if I read it correctly.
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