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Upcoming SchedulePSU Stat 2014Monday Tuesday Wednesday Thursday Friday
Jan 6Sec 7.2
Jan 7 Jan 8Sec 7.3
Jan 9 Jan 10Sec 7.4
Jan 13
Chapter 7 in a nutshell
Jan 14 Jan 15
Chapter 7 test
Jan 16 Jan 17
Final Review
MLK Jr DayNo School
Jan 21
Monday schedule
Final Review
Jan 22
Final Per 1-3
Jan 23
Final Per 4-6
Jan 24
Final Per 7,8
ReviewTerm SymbolSample standard deviation s
Population standard deviation s
Sample Variance s2
Population Variance s2
Bluman, Chapter 7 2
7-4 Confidence Intervals for Variances and Standard Deviations
When products that fit together (such as pipes) are manufactured, it is important to keep the variations of the diameters of the products as small as possible; otherwise, they will not fit together properly and will have to be scrapped.
In the manufacture of medicines, the variance and standard deviation of the medication in the pills play an important role in making sure patients receive the proper dosage.
For these reasons, confidence intervals for variances and standard deviations are necessary.
Bluman, Chapter 7 3
Chi-Square Distributions The chi-square distribution must be used to calculate
confidence intervals for variances and standard deviations.
The chi-square variable is similar to the t variable in that its distribution is a family of curves based on the number of degrees of freedom.
The symbol for chi-square is (Greek letter chi, pronounced “ki”).
A chi-square variable cannot be negative, and the distributions are skewed to the right.
Bluman, Chapter 7 4
2
Chi-Square Distributions
Bluman, Chapter 7 5
At about 100 degrees of freedom, the chi-square distribution becomes somewhat symmetric.
The area under each chi-square distribution is equal to 1.00, or 100%.
Formula for the Confidence Interval for a Variance; see page 388
Bluman, Chapter 7 6
2 22
2 2right left
1 1, d.f. = 1
n s n sns
Bluman, Chapter 7 7
2 2
2 2right left
1 1, d.f. = 1
n s n sns
Formula for the Confidence Interval for a Standard Deviation
Rounding Rule: Please refer to the bottom of page 388.
Chapter 7Confidence Intervals and Sample Size
Section 7-4Example 7-13Page #387
Bluman, Chapter 7 8
Find the values for and for a 90% confidence interval when n = 25.
Example 7-13: Using Table G
Bluman, Chapter 7 9
To find , subtract 1 - 0.90 = 0.10. Divide by 2 to get 0.05.To find , subtract 1 - 0.05 to get 0.95.
2left2
right
2right2left
Use the 0.95 and 0.05 columns and the row corresponding to 24 d.f. in Table G.
Example 7-13: Using Table G
Bluman, Chapter 7 10
2 2right leftThe value is 36.415; the value is 13.848.
Using table G Generalized:
11
1) Calculate a which is the complement of the CL.2) chi square right: 3) chi square left: subtract the above value from 1
Rounding Rule
When you are computing a confidence interval for a population variance or standard deviation by using raw data, round off to one more decimal places than the number of decimal places in the original data.
When you are computing a confidence interval for a population variance or standard deviation by using a sample variance or standard deviation, round off to the same number of decimal places as given for the sample variance or standard deviation.
Confidence Interval for a Variance or Standard Deviation
Bluman, Chapter 7 12
Rounding Rule
When you are computing a confidence interval for a population variance or standard deviation by using raw data, round off to one more decimal places than the number of decimal places in the original data.
When you are computing a confidence interval for a population variance or standard deviation by using a sample variance or standard deviation, round off to the same number of decimal places as given for the sample variance or standard deviation.
Confidence Interval for a Variance or Standard Deviation
Bluman, Chapter 7 13
Chapter 7Confidence Intervals and Sample Size
Section 7-4Example 7-14Page #389
Bluman, Chapter 7 14
Find the 95% confidence interval for the variance and standard deviation of the nicotine content of cigarettes manufactured if a sample of 20 cigarettes has a standard deviation of 1.6 milligrams.
Example 7-14: Nicotine Content
Bluman, Chapter 7 15
To find , subtract 1 - 0.95 = 0.05. Divide by 2 to get 0.025.
To find , subtract 1 - 0.025 to get 0.975.
In Table G, the 0.025 and 0.975 columns with the d.f. 19 row yield values of 32.852 and 8.907, respectively.
2right
2left
Example 7-14: Nicotine Content
Bluman, Chapter 7 16
2 22
2 2right left
1 1
n s n ss
21.5 5.5 s
1.5 5.51.2 2.3
ss
You can be 95% confident that the true variance for the nicotine content is between 1.5 and 5.5 milligrams.
2 2219 1.6 19 1.6
32.852 8.907 s
You can be 95% confident that the true standard deviation is between 1.2 and 2.3 milligrams.
Chapter 7Confidence Intervals and Sample Size
Section 7-4Example 7-15Page #389
Bluman, Chapter 7 17
Find the 90% confidence interval for the variance and standard deviation for the price in dollars of an adult single-day ski lift ticket. The data represent a selected sample of nationwide ski resorts. Assume the variable is normally distributed.
59 54 53 52 5139 49 46 49 48
Example 7-15: Cost of Ski Lift Tickets
Bluman, Chapter 7 18
Using technology, we find the variance of the data is approximately s2=28.2; however use the VARS key to use the exact value in the formula.
In Table G, the 0.05 and 0.95 columns with the d.f. 9 row yield values of 16.919 and 3.325, respectively.
Example 7-15: Cost of Ski Lift Tickets
19
2 22
2 2right left
1 1
n s n ss
You can be 95% confident that the true variance for the cost of ski lift tickets is between 15.0 and 76.4.
29 28.2 9 28.216.919 3.325
s
15.0<𝜎2<76.4
Example 7-15: Cost of Ski Lift Tickets
Bluman, Chapter 7 20
You can be 95% confident that the true standard deviation is between $3.9 and $8.7.
15.0<𝜎2<76.4To determine the interval for the standard deviation, don’t use the rounded answers of the variation. Use the exact values.
√15.0<𝜎<√76.43.9<𝜎<8.7
Homework
Sec 7.4 page 390 #5,8,9,11
Bluman, Chapter 7 21
Bluman, Chapter 7 22
Confidence Level
2 leftColumn to use
2 rigthColumn to use
90% 0.95 0.0595% 0.975 0.02598% 0.99 0.0199% 0.995 0.005