1. Variance and Standard Deviation
Chapter 3.2
Variance and Standard Deviation

2. Comparison of Outdoor Paint
A testing lab wishes to test two experimental brands of outdoor paint to see how long each will last before fades. The testing lab makes 6 gallons of each paint to test. Since different chemical agents are added to each group and only six cans involved, these two groups constitute two small populations. The results in months are shown.
Brand A
Brand B 10 35 60 45 50 30 40 20 25

3. Find the mean for Brand A and Brand B
Recall formula for Mean

4. Results?
Although the means are the same, we cannot conclude that both brands of paint last equally well.
Find the range:

5. Definitions and formulas for variability of a data set:
The variance is the average of the squares of the distance of each value is from the mean.
The standard deviation is the square root of the variance.
X = individual value, N = population size,
µ = population mean

6. Steps for finding Variance and Standard Deviation
Find the mean of the data
Subtract the mean from each data value
Square each result
Find the sum of the squares
Divide the sum by N to get the variance
Take the square root of the variance to get the standard deviation
(It might be helpful to organize the data in a table)

7. Find the Standard Deviation and Variance of both brands of paint
Brand A
Brand B
Values, X
X - µ
(X - µ)2 10 35 60 45 50 30 40 20 25

8. Conclusions
Since the standard deviation of brand A is greater than the standard deviation of brand B, the data are more variable for brand A.
In summary, when the means are equal, the larger the variance or standard deviation is, the more variable the data are.

9. Formulas for samples
Variance:
Standard Deviation:

10. Steps for finding the sample variance and standard deviation:
Find the sum of the values (∑X)
Square each value and find the sum (∑X2)
Substitute in the formulas and solve

11. Example:
Find the sample variance and standard deviation for the amount of European auto sales for a sample of 6 years shown. The data are in millions of dollars.
11.2, 11.9, 12.0, 12.8, 13.4, 14.3

12. Precipitation and High Temperatures
The normal daily high temperatures (in degrees Fahrenheit) in January for 10 selected cities are as follows: 50, 37, 29, 54, 30, 61, 47, 38, 34, 61
The normal monthly precipitation (in inches) for these same 10 cities is listed here:
4.8, 2.6, 1.5, 1.8, 1.8, 3.3, 5.1, 1.1, 1.8, 2.5
Which set is more variable?