The PEFR of a group of 11 year old girls follow a normal distribution with mean 300 L/min and stardard detion 20 L/min-
**Core Concept**
The question tests understanding of a normal distribution, specifically how mean and standard deviation relate to percentiles in a population. In a normal distribution, the mean, median, and mode are equal, and the standard deviation is a measure of variability.
**Why the Correct Answer is Right**
Given that the PEFR (Peak Expiratory Flow Rate) follows a normal distribution with a mean of 300 L/min and a standard deviation of 20 L/min, we can use the properties of the normal distribution to understand where most values lie. About 68% of values fall within 1 standard deviation of the mean, and about 95% fall within 2 standard deviations.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not relate to the specific parameters of the normal distribution provided.
**Option B:** Incorrect as it does not correctly apply the principles of normal distribution to the given mean and standard deviation.
**Option C:** Incorrect because it does not accurately reflect how percentiles are distributed in a normal distribution.
**Option D:** This is the correct answer but the explanation for why the other options are incorrect is based on the principle that the correct answer choice reflects an accurate application of normal distribution principles.
**Clinical Pearl / High-Yield Fact**
Remembering that in a normal distribution, about 95% of values lie within 2 standard deviations of the mean is crucial for understanding and interpreting data in medical research and practice.
**Correct Answer:** D.