1. Unit 3 Lesson 2 (4.2) Numerical Methods for Describing Data
4.2: Describing Variability in a Data Set

2. Why is the study of variability important?
Does this can of soda contain exactly 12 ounces?
There is variability in virtually everything
Reporting only a measure of center doesn’t provide a complete picture of the distribution.

3. Notice that these three data sets all have the same mean and median (at 45), but they have very different amounts of variability.

4. Measures of Variability
The simplest numeric measure of variability is range.
Range = largest observation – smallest observation
The first two data sets have a range of 50 (70-20) but the third data set has a much smaller range of 10.

5. Measures of Variability
Remember the sample of 6 fish that we caught from the lake . . .
They were the following lengths:
3”, 4”, 5”, 6”, 8”, 10”
The mean length was 6 inches. We can calculate the deviations from the mean.
Another measure of the variability in a data set uses the deviations from the mean (x – x).

6. YES Now find how each observation deviates from the mean.
The mean is considered the balance point of the distribution because it “balances” the positive and negative deviations.
This is the deviation from the mean. x
(x - x) 3 4 5 6 8 10
Sum
3-6 -3 -2
Find the rest of the deviations from the mean -1
What is the sum of the deviations from the mean? 2
Will this sum always equal zero? 4
YES

7. Measures of Variability
Population variance is denoted by s2.
Remember the sample of 6 fish that we caught from the lake . . .
They were the following lengths:
3”, 4”, 5”, 6”, 8”, 10”
The mean length was 6 inches. We can calculate the deviations from the mean. What’s the sum of these deviations?
Can we find an average deviation?
The estimated average of the deviations squared is called the variance.

8. Remember the sample of 6 fish that we caught from the lake . . .
Find the variance of the length of fish.
First square the deviations x
(x - x)
(x - x)2 3 -3 4 -2 5 -1 6 8 2 10
Sum
Finding the average of the deviations would always equal 0! 9 4 1 16
What is the sum of the deviations squared?
Divide this by 5. 34
s2 = 6.5

9. Measures of Variability
The square root of the variance is called standard deviation.
Standard Deviation: the average difference from the mean
s2 = 6.8 inches2 so s = inches
The fish in our sample deviate from the mean of 6 by an average of inches.

10. Calculation of standard deviation of a sample
The most commonly used measures of center and variability are the mean and standard deviation, respectively.
Calculation of standard deviation of a sample
Population standard deviation is denoted by s.

11. Measures of Variability
Interquartile range (IQR) is the range of the middle half of the data.
Lower quartile (Q1) is the median of the lower half of the data
Upper quartile (Q3) is the median of the upper half of the data
IQR = Q3 – Q1
What advantage does the interquartile range have over the standard deviation?
The IQR is resistant to extreme values

12. Find the interquartile range for this set of data.
The Chronicle of Higher Education ( issue) published the accompanying data on the percentage of the population with a bachelor’s or higher degree in 2007 for each of the 50 states and the District of Columbia.
Find the interquartile range for this set of data.

13. First put the data in order & find the median. 24 26 30
First put the data in order & find the median.
Find the lower quartile (Q1) by finding the median of the lower half.
Find the upper quartile (Q3) by finding the median of the upper half.
IQR = 30 – 24 = 6

14. Homework
Pg.120: #4.19, 4.21, 4.22, 4.26