Table of contents
- 1. Intro to Stats and Collecting Data55m
- 2. Describing Data with Tables and Graphs1h 55m
- 3. Describing Data Numerically1h 45m
- 4. Probability2h 16m
- 5. Binomial Distribution & Discrete Random Variables2h 33m
- 6. Normal Distribution and Continuous Random Variables1h 38m
- 7. Sampling Distributions & Confidence Intervals: Mean1h 3m
- 8. Sampling Distributions & Confidence Intervals: Proportion1h 12m
- 9. Hypothesis Testing for One Sample1h 1m
- 10. Hypothesis Testing for Two Samples2h 8m
- 11. Correlation48m
- 12. Regression1h 4m
- 13. Chi-Square Tests & Goodness of Fit1h 20m
- 14. ANOVA1h 0m
6. Normal Distribution and Continuous Random Variables
Standard Normal Distribution
Problem 6.CR.3c
Textbook Question
Foot Lengths of Women Assume that foot lengths of adult females are normally distributed with a mean of 246.3 mm and a standard deviation of 12.4 mm (based on Data Set 3 “ANSUR II 2012†in Appendix B).
c. Find P95.

1
Step 1: Understand the problem. The question asks for P95, which represents the 95th percentile of a normal distribution. This means we need to find the value of foot length (x) such that 95% of the data lies below it. The distribution is normal with a mean (μ) of 246.3 mm and a standard deviation (σ) of 12.4 mm.
Step 2: Recall the formula for converting a raw score (x) to a z-score in a normal distribution: z = (x - μ) / σ. To find P95, we will reverse this formula to solve for x: x = z * σ + μ.
Step 3: Look up the z-score corresponding to the 95th percentile in a standard normal distribution table or use statistical software. The z-score for the 95th percentile is approximately 1.645.
Step 4: Substitute the known values into the formula x = z * σ + μ. Here, z = 1.645, μ = 246.3 mm, and σ = 12.4 mm. The formula becomes x = (1.645 * 12.4) + 246.3.
Step 5: Simplify the expression to calculate the value of x, which represents P95. This will give the foot length at the 95th percentile.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Normal Distribution
Normal distribution is a probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean. It is characterized by its bell-shaped curve, defined by its mean and standard deviation. In this context, foot lengths of women are assumed to follow a normal distribution, which allows for the application of statistical methods to analyze the data.
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Percentiles
A percentile is a measure used in statistics to indicate the value below which a given percentage of observations fall. For example, P95 (the 95th percentile) represents the foot length below which 95% of the adult female population's foot lengths fall. Understanding percentiles is crucial for interpreting data distributions and making comparisons within a dataset.
Z-scores
A Z-score is a statistical measurement that describes a value's relationship to the mean of a group of values, expressed in terms of standard deviations. It is calculated by subtracting the mean from the value and then dividing by the standard deviation. Z-scores are essential for finding percentiles in a normal distribution, as they allow us to determine how far a specific value is from the mean and to use standard normal distribution tables.
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