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
3. Describing Data Numerically
Mean
Problem 2.3.52
Textbook Question
Finding the Mean of a Frequency Distribution In Exercises 49–52, approximate the mean of the frequency distribution.
Populations The populations (in thousands) of the counties in Montana in 2019 (Source: U.S. Census Bureau)


1
Step 1: Identify the midpoints of each population interval. The midpoint is calculated as the average of the lower and upper boundaries of each interval. For example, for the interval 0–19, the midpoint is (0 + 19) / 2 = 9.5.
Step 2: Multiply each midpoint by its corresponding frequency to find the 'weighted total' for each interval. For example, for the interval 0–19, the weighted total is 9.5 * 46.
Step 3: Sum all the weighted totals obtained in Step 2. This gives the total of all midpoints weighted by their frequencies.
Step 4: Sum all the frequencies from the table. This gives the total number of counties.
Step 5: Divide the sum of the weighted totals (from Step 3) by the sum of the frequencies (from Step 4). This quotient is the approximate mean of the frequency distribution.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Mean of a Frequency Distribution
The mean of a frequency distribution is calculated by multiplying each population midpoint by its corresponding frequency, summing these products, and then dividing by the total number of observations. This provides a weighted average that reflects the distribution of data across different population ranges.
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Midpoint Calculation
The midpoint of a class interval is found by averaging the lower and upper boundaries of the interval. For example, the midpoint for the population range 0-19 is (0 + 19) / 2 = 9.5. This value is essential for calculating the mean, as it represents a single value for each range that can be used in the mean formula.
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Total Frequency
Total frequency is the sum of all frequencies in a frequency distribution. It represents the total number of observations or data points being analyzed. In this case, it is crucial for determining the denominator when calculating the mean, ensuring that the mean reflects the average population across all counties.
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