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Review Question - QID 102598

QID 102598 (Type "102598" in App Search)
A study on cholesterol levels of a town in rural Idaho is performed, of which there are 1000 participants. It is determined that in this population, the mean LDL is 200 mg/dL, with a standard deviation of 50 mg/dL. If the population has a normal distribution, how many people have a cholesterol less than 300 mg/dL?

680

4%

6/138

925

5%

7/138

950

22%

30/138

975

59%

81/138

997

3%

4/138

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In a normally distributed sample, 95% of the observations are expected to fall within 2 standard deviations of the mean (950 people); however, when asked how many observations are below 2 standard deviations above the mean, 2.5% of the population less than 2 standard deviations (1/2 of the 5% not included in the 2 standard deviation population) would also be included (total of 975 people).

The utility of using normal curves in biostatistics is the ability to rely on certain characteristics of the normal distribution (see Rosner, below). Since this sample's standard deviation is 50 mg/dL, the values of 100 and 300 mg/dL are each 2 standard deviations above and below the mean of 200 mg/dL.

Rosner defines one property of the normal distribution as such: 68%, 95%, and 99.7% of observations can be expected to fall within 1, 2, and 3 standard deviations respectively of the mean.

Illustration A displays a normal statistical distribution.

Incorrect answers:
Answer 1: 680 people fall within 1 standard deviation of the mean.
Answer 2: 925 people does not represent all patients greater than 2 standard deviations above the mean.
Answer 3: 950 people fall within 2 standard deviations of the mean
Answer 5: 997 people fall within 3 standard deviations of the mean.

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