# math 3680 lecture #15 confidence intervals

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Math 3680 Lecture #15 Confidence Intervals. Review: Suppose that E( X ) = m and SD( X ) = s . Recall the following two facts about the average of n observations drawn with replacement:. Estimation. - PowerPoint PPT PresentationTRANSCRIPT

Math 3680

Lecture #15

Confidence Intervals

Review: Suppose that E(X) = m and SD(X) = s. Recall the following two facts about the average of n observations drawn with replacement:

Estimation

Example: A university has 25,000 registered students. In a survey of 318 students, the average age of the sample is found to be 22.4, with a sample SD of 4.5 years. Estimate the average age of all 25,000 students, and attach a standard error to this estimate.

Wrong Answer: The average age of thestudent body is exactly 22.4 years.What is wrong with this simplistic analysis?

Answer: Of course, we estimate the average of the population to be 22.4 years but this estimate will not be exact. To determine the magnitude of the error, we need to find the SE, and that means a box model. 25,000 tickets Average = ?? SD = ?? 318 draws

Bootstrap Estimation: Although the SD of the box is unknown, we estimate the SD of the box from the fractions in the sample:

SD of box 4.5SE of the sample average

(Why?)

Conclusion: The average age is about 22.4 years, give or take 0.251 years or so.

Confidence Intervals:Large samples or known s

68%We say that the range

22.40.251 years = 22.149-22.651 years

is a 68% confidence interval for the average age of the population.

95%We say that the range

22.4(1.96)(0.251) years = 21.909-22.891 years

is a 95% confidence interval for the average age of the population.

99.7%We say that the range

22.4(2.968)(0.251) years = 21.656-23.144 years

is a 99.7% confidence interval for the average age of the population.

1 - 2aIn general, we say that the range

is a 1 - 2a confidence interval for the population average m.zaz1-a

Logic:

Observations:

1) We are NOT saying that 95% of the students are between 21.9 and 22.9 years old this is patently ridiculous, of course.

2) We are NOT saying that there is a 95% chance that the average age is between 21.9 and 22.9 years. The population average is constant it is either in this range or it is not.

Observations:

3) The true interpretation is as follows: If several people run this experiment and they all find a 95%-confidence interval, then the true population parameter will lie in about 95% of these intervals.

100 different 95% confidence intervals

100 different 68% confidence intervals

100 different 95% confidence intervals, n = 4 x 318 =1272

Observations:

4) In the previous problem, we replaced the population s with the sample s. (When did we do this?) As it turns out, this makes little practical difference for large samples.

More on this later when we consider small samples.

Observations:5) The normal approximation has been used. As discussed earlier, a large number of draws is required for this assumption to hold.

6) Remember: There is no such thing as a 100% confidence interval. In practice, scientists often use 95% as a balance between a high confidence level and a narrow confidence interval.

Example: In a simple random sample of 680 households (in a city of millions), the average number of TV sets is 1.86, with an SD of 0.80. Find a 95% confidence interval for the average number of TV sets per household in the city.

True or false:(i) 1.860.06 is a 95%-confidence interval for this population average.(ii) 1.860.06 is a 95%-confidence interval for this sample average.

(iii) There is a 95% chance for the population average to be in the range 1.860.06.

Example: The chart to the right shows platelet counts among 120 geriatric patients. Find a 95% confidence interval for the average platelet count among geriatric patients.

Sheet1

132117176126142120

127125198208105146

214194131208101139

184163129138110247

181181125123117176

211108254244139179

190212228139147170

139129174108106141

112126125142115147

105256142175131119

174106194181196232

143142104184112141

135110107137111112

185114188106102104

120143179178124242

235129198150180187

142125184238111129

129203115101178133

134168229169148185

154162103105125151

Sheet2

Sheet3

Fill in the blanks with either box or draws.

Probabilities are used when reasoning from the __________ to the _____________.

Confidence levels are used when reasoning from the ____________ to the ______________.

Fill in the blank with either observed or expected.The chance error is in the _______________ value.

Fill in the blank with either sample or population.The confidence level is for the ______________ average.

Confidence Intervals:Projecting Sample Size

Example: In a preliminary simple random sample of 680 households (in a city of millions), the average number of TV sets in the sample households is 1.86, with an SD of 0.80.

Suppose that its desired to construct a 90% confidence interval which has a margin of error of 0.03. How large a sample would be necessary?

Solution:So, the sample size should be at least 1925

Confidence Intervals:Small samples

Example: A biological research team measures the weights of 14 chipmunks, randomly chosen. Find a 90% confidence interval for the average weight of chipmunks.

Sheet1

7.68.669.418.458.088.867.488.7642857143

8.29.249.349.5810.18.559.150.7610389082

Sheet2

Sheet3

Note: The previous calculations used the fact that

approximately follows the normal curve for large values of n. In this problem, we cannot use this approximation.

However, for both small and large samples, we can use the fact that

approximately follows the Students t-distribution with n - 1 degrees of freedom.

1 - 2aIn general, we say that the range

is a 1 - 2a confidence interval for the population average m.tn-1, atn-1, 1-a

90%Therefore, the 90% confidence interval is

or 8.40 9.12 ounces.Excel:TINV(0.1, 13)

Note: Be sure you look up the correct number on the table in the back of the book. The numbers at the bottom of Table 4 specify the two-sided confidence levels.

Excel 2003: Place all data values in a single row or column. Tools -> Data Analysis -> Descriptive Statistics Select the data range, and check columns or rows, Confidence Level for Mean and Summary Statistics.

Sheet5

Row1

Mean8.4378571429

Standard Error0.1177778621

Median8.31

Mode

Standard Deviation0.4406844078

Sample Variance0.1942027473

Kurtosis0.291035133

Skewness0.9319878034

Range1.56

Minimum7.84

Maximum9.4

Sum118.13

Count14

Confidence Level(95.0%)0.2544435527

Sheet1

7.987.848.338.178.158.299.098.168.348.548.748.239.48.87

Sheet2

Sheet3

Excel 2007

First, to get started

From the circle, select Excel Options. Click Add-Ins. Next to Manage Excel Add-Ins, click Go. Check Analysis ToolPak, and click OK.

Excel 2007 Place all data values in a single row or column. Click Descriptive Statistics. Select the data range, and check columns or rows, Confidence Level for Mean and Summary Statistics.

Sheet5

Row1

Mean8.4378571429

Standard Error0.1177778621

Median8.31

Mode

Standard Deviation0.4406844078

Sample Variance0.1942027473

Kurtosis0.291035133

Skewness0.9319878034

Range1.56

Minimum7.84

Maximum9.4

Sum118.13

Count14

Confidence Level(95.0%)0.2544435527

Sheet1

7.987.848.338.178.158.299.098.168.348.548.748.239.48.87

Sheet2

Sheet3

Notes: The confidence level is really the margin of error. The last two rows have to be entered by hand.

Sheet5

Row1

Mean8.4378571429

Standard Error0.1177778621

Median8.31

Mode

Standard Deviation0.4406844078

Sample Variance0.1942027473

Kurtosis0.291035133

Skewness0.9319878034

Range1.56

Minimum7.84

Maximum9.4

Sum118.13

Count14

Confidence Level(95.0%)0.2544435527

Sheet1

7.987.848.338.178.158.299.098.168.348.548.748.239.48.87

Sheet2

Sheet3

Sheet6

Row1

Mean8.4378571429

Standard Error0.1177778621

Median8.31

Mode

Standard Deviation0.4406844078

Sample Variance0.1942027473

Kurtosis0.291035133

Skewness0.9319878034

Range1.56

Minimum7.84

Maximum9.4

Sum118.13

Count14

Confidence Level(90.0%)0.20857655

Mean - Margin of Error8.2292805929

Mean + Margin of Error8.6464336928

Sheet1

7.987.848.338.178.158.299.098.168.348.548.748.239.48.87

Sheet2

Sheet3

Example: Duracell tests 12 batteries in flashlights. They determine that the average life of the batteries in this sample is 3.58 hours, with a sample SD of 1.58 hours. Find a 95% confidence interval for the average life of a Duracell battery in a flashlight.

Repeat if 100 batteries were tested (with the same sample mean and SD as above)

Note: In previous lectures, we considered another technique of inferring information about the box from the draws namely, hypothesis testing.Confidence intervals provide a method of estimating the average of the box.Hypothesis testing checks if the difference between the

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