SOLUTION: Suppose the distribution of percentage of ideal body weight in a group of women with insulin dependent diabetes mellitus is normal with a mean of 175 lbs and standard deviation of

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Question 795384: Suppose the distribution of percentage of ideal body weight in a group of women with insulin dependent diabetes mellitus is normal with a mean of 175 lbs and standard deviation of 13.
My question is:
What body weight is considered obese (obese is defined as the top 20% of the subjects) and what body weight is considered underweight (defined as in the bottom 10% for body weight)?
I've tried using the z-score approach with the normal distribution tables however I can't seem to figure it out.
Answer by stanbon(75887) (Show Source):
You can put this solution on YOUR website!
Suppose the distribution of percentage of ideal body weight in a group of women with insulin dependent diabetes mellitus is normal with a mean of 175 lbs and standard deviation of 13.
My question is:
What body weight is considered obese (obese is defined as the top 20% of the subjects)
Find the z-value with a left tail of 80%::
invNorm(0.8) = 0.8416
Find the corresponding weight value::
x = z*s + u
x = 0.8416*13+175 = 185.94 lbs.
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and what body weight is considered underweight (defined as in the bottom 10% for body weight)?
Find the z-value with left tail of 10%::
invNorm(0.1) = -1.2816
x = -1.2816*13+175 = 158.34 lbs
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Cheers,
Stan H.