Unit 6 Confidence Intervals

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Point Estimate for Population μ

Point Estimate

• A single value estimate for a population parameter

• Most unbiased point estimate of the population mean μ is the sample mean x

Estimate Population Parameter…

with Sample Statistic

Mean: μ x

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Example: Point Estimate for Population μ

Market researchers use the number of sentences per advertisement as a measure of readability for magazine advertisements. The following represents a random sample of the number of sentences found in 50 advertisements. Find a point estimate of the population mean, . (Source: Journal of Advertising Research)

9 20 18 16 9 9 11 13 22 16 5 18 6 6 5 12 2517 23 7 10 9 10 10 5 11 18 18 9 9 17 13 11 714 6 11 12 11 6 12 14 11 9 18 12 12 17 11 20

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Solution: Point Estimate for Population μ

The sample mean of the data is

62012.4

50

xx

n

Your point estimate for the mean length of all magazine advertisements is 12.4 sentences.

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Interval Estimate

Interval estimate

• An interval, or range of values, used to estimate a population parameter.

Point estimate

• 12.4

How confident do we want to be that the interval estimate contains the population mean μ?

( )

Interval estimate

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Level of Confidence

Level of confidence c

• The probability that the interval estimate contains the population parameter.

zz = 0-zc zc

Critical values

½(1 – c) ½(1 – c)

c is the area under the standard normal curve between the critical values.

The remaining area in the tails is 1 – c .

c

Use the Standard Normal Table to find the corresponding z-scores.

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zc

Level of Confidence

• If the level of confidence is 90%, this means that we are 90% confident that the interval contains the population mean μ.

zz = 0 zc

The corresponding z-scores are +1.645.

c = 0.90

½(1 – c) = 0.05½(1 – c) = 0.05

-zc = -1.645 zc = 1.645

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Sampling Error

Sampling error

• The difference between the point estimate and the actual population parameter value.

• For μ: the sampling error is the difference – μ μ is generally unknown varies from sample to sample

x

x

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Margin of Error

Margin of error

• The greatest possible distance between the point estimate and the value of the parameter it is estimating for a given level of confidence, c.

• Denoted by E.

• Sometimes called the maximum error of estimate or error tolerance.

c x cE z zn

σσ When n 30, the sample standard deviation, s, can be used for .

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Example: Finding the Margin of Error

Use the magazine advertisement data and a 95% confidence level to find the margin of error for the mean number of sentences in all magazine advertisements. Assume the sample standard deviation is about 5.0.

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zc

Solution: Finding the Margin of Error

• First find the critical values

zzcz = 0

0.95

0.0250.025

-zc = -1.96

95% of the area under the standard normal curve falls within 1.96 standard deviations of the mean. (You can approximate the distribution of the sample means with a normal curve by the Central Limit Theorem, because n ≥ 30.)

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zc = 1.96

Solution: Finding the Margin of Error

5.01.96

501.4

c c

sE z z

n n

You don’t know σ, but since n ≥ 30, you can use s in place of σ.

You are 95% confident that the margin of error for the population mean is about 1.4 sentences.

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Confidence Intervals for the Population Mean

A c-confidence interval for the population mean μ

•

• The probability that the confidence interval contains μ is c.

where cx E x E E zn

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Constructing Confidence Intervals for μ

Finding a Confidence Interval for a Population Mean (n 30 or σ known with a normally distributed population)

In Words In Symbols

1. Find the sample statistics n and .

2. Specify , if known. Otherwise, if n 30, find the sample standard deviation s and use it as an estimate for .

xxn

2( )1

x xsn

x

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Constructing Confidence Intervals for μ

3. Find the critical value zc that corresponds to the given level of confidence.

4. Find the margin of error E.

5. Find the left and right endpoints and form the confidence interval.

Use the Standard Normal Table.

Left endpoint: Right endpoint: Interval:

cE zn

x Ex E

x E x E

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In Words In Symbols

Example: Constructing a Confidence Interval

Construct a 95% confidence interval for the mean number of sentences in all magazine advertisements.

Solution: Recall and E = 1.412.4x

12.4 1.4

11.0

x E

12.4 1.4

13.8

x E

11.0 < μ < 13.8

Left Endpoint: Right Endpoint:

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( )

Solution: Constructing a Confidence Interval

11.0 < μ < 13.8

• 12.411.0 13.8

With 95% confidence, you can say that the population mean number of sentences is between 11.0 and 13.8.

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Example: Constructing a Confidence Interval σ Known

A college admissions director wishes to estimate the mean age of all students currently enrolled. In a random sample of 20 students, the mean age is found to be 22.9 years. From past studies, the standard deviation is known to be 1.5 years, and the population is normally distributed. Construct a 90% confidence interval of the population mean age.

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zc

Solution: Constructing a Confidence Interval σ Known

• First find the critical values

zz = 0 zc

c = 0.90

½(1 – c) = 0.05½(1 – c) = 0.05

-zc = -1.645 zc = 1.645

zc = 1.645

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• Margin of error:

• Confidence interval:

Solution: Constructing a Confidence Interval σ Known

1.51.645 0.6

20cE z

n

22.9 0.6

22.3

x E

22.9 0.6

23.5

x E

Left Endpoint: Right Endpoint:

22.3 < μ < 23.5

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Solution: Constructing a Confidence Interval σ Known

22.3 < μ < 23.5

( )• 22.922.3 23.5

With 90% confidence, you can say that the mean age of all the students is between 22.3 and 23.5 years.

Point estimate

xx E x E

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Interpreting the Results

• μ is a fixed number. It is either in the confidence interval or not.

• Incorrect: “There is a 90% probability that the actual mean is in the interval (22.3, 23.5).”

• Correct: “If a large number of samples is collected and a confidence interval is created for each sample, approximately 90% of these intervals will contain μ.

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Sample Size

• Given a c-confidence level and a margin of error E, the minimum sample size n needed to estimate the population mean is

• If is unknown, you can estimate it using s provided you have a preliminary sample with at least 30 members.

2

czn

E

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Example: Sample Size

You want to estimate the mean number of sentences in a magazine advertisement. How many magazine advertisements must be included in the sample if you want to be 95% confident that the sample mean is within one sentence of the population mean? Assume the sample standard deviation is about 5.0.

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zc

Solution: Sample Size

• First find the critical values

zc = 1.96

zz = 0 zc

0.95

0.0250.025

-zc = -1.96

25

zc = 1.96

Solution: Sample Size

zc = 1.96 s = 5.0 E = 1

221.96 5.0

96.041

czn

E

When necessary, round up to obtain a whole number.

You should include at least 97 magazine advertisements in your sample.

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Section 6.2 Objectives

• Interpret the t-distribution and use a t-distribution table

• Construct confidence intervals when n < 30, the population is normally distributed, and σ is unknown

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The t-Distribution

• When the population standard deviation is unknown, the sample size is less than 30, and the random variable x is approximately normally distributed, it follows a t-distribution.

• Critical values of t are denoted by tc.

-xtsn

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Properties of the t-Distribution

1. The t-distribution is bell shaped and symmetric about the mean.

2. The t-distribution is a family of curves, each determined by a parameter called the degrees of freedom. The degrees of freedom are the number of free choices left after a sample statistic such as is calculated. When you use a t-distribution to estimate a population mean, the degrees of freedom are equal to one less than the sample size. d.f. = n – 1 Degrees of freedom

x

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Properties of the t-Distribution

3. The total area under a t-curve is 1 or 100%.

4. The mean, median, and mode of the t-distribution are equal to zero.

5. As the degrees of freedom increase, the t-distribution approaches the normal distribution. After 30 d.f., the t-distribution is very close to the standard normal z-distribution.

t0

Standard normal curve

The tails in the t-distribution are “thicker” than those in the standard normal distribution.d.f. = 5

d.f. = 2

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Example: Critical Values of t

Find the critical value tc for a 95% confidence when the sample size is 15.

Table 5: t-Distribution

tc = 2.145

Solution: d.f. = n – 1 = 15 – 1 = 14

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Solution: Critical Values of t

95% of the area under the t-distribution curve with 14 degrees of freedom lies between t = +2.145.

t

-tc = -2.145 tc = 2.145

c = 0.95

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Confidence Intervals for the Population Mean

A c-confidence interval for the population mean μ

•

• The probability that the confidence interval contains μ is c.

where c

sx E x E E t

n

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Confidence Intervals and t-Distributions

1. Identify the sample statistics n, , and s.

2. Identify the degrees of freedom, the level of confidence c, and the critical value tc.

3. Find the margin of error E.

xxn

2( )1

x xsn

cE tn

s

d.f. = n – 1

x

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In Words In Symbols

Confidence Intervals and t-Distributions

4. Find the left and right endpoints and form the confidence interval.

Left endpoint: Right endpoint: Interval:

x Ex E

x E x E

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In Words In Symbols

Example: Constructing a Confidence Interval

You randomly select 16 coffee shops and measure the temperature of the coffee sold at each. The sample mean temperature is 162.0ºF with a sample standard deviation of 10.0ºF. Find the 95% confidence interval for the mean temperature. Assume the temperatures are approximately normally distributed.

Solution:Use the t-distribution (n < 30, σ is unknown, temperatures are approximately distributed.)

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Solution: Constructing a Confidence Interval

• n =16, x = 162.0 s = 10.0 c = 0.95

• df = n – 1 = 16 – 1 = 15

• Critical Value

Table 5: t-Distribution

tc = 2.131

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Solution: Constructing a Confidence Interval

• Margin of error:

• Confidence interval:102.131 5.316cE t

n s

162 5.3

156.7

x E

162 5.3

167.3

x E

Left Endpoint: Right Endpoint:

156.7 < μ < 167.3

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Solution: Constructing a Confidence Interval

• 156.7 < μ < 167.3

( )• 162.0156.7 167.3

With 95% confidence, you can say that the mean temperature of coffee sold is between 156.7ºF and 167.3ºF.

Point estimate

xx E x E

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No

Normal or t-Distribution?

Is n 30?

Is the population normally, or approximately normally, distributed? Cannot use the normal

distribution or the t-distribution. Yes

Is known?

No

Use the normal distribution with

If is unknown, use s instead.

cE zn

σYes

No

Use the normal distribution with

cE zn

σ

Yes

Use the t-distribution with

and n – 1 degrees of freedom.

cE tn

s

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Section 6.3 Point Estimate for Population p

Population Proportion

• The probability of success in a single trial of a binomial experiment.

• Denoted by p

Point Estimate for p

• The proportion of successes in a sample.

• Denoted by read as “p hat”

number of successes in sampleˆ number in samplexpn

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Point Estimate for Population p

Point Estimate for q, the proportion of failures

• Denoted by

• Read as “q hat”

1ˆ ˆq p

Estimate Population Parameter…

with Sample Statistic

Proportion: p p̂

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Example: Point Estimate for p

In a survey of 1219 U.S. adults, 354 said that their favorite sport to watch is football. Find a point estimate for the population proportion of U.S. adults who say their favorite sport to watch is football. (Adapted from The Harris Poll)

Solution: n = 1219 and x = 354

354 0.29ˆ 0402 29.0%1219

xpn

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Confidence Intervals for p

A c-confidence interval for the population proportion p •

•The probability that the confidence interval contains p is c.

ˆ ˆwhereˆ ˆ cpqp E p p E E zn

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Constructing Confidence Intervals for p

1. Identify the sample statistics n and x.

2. Find the point estimate

3. Verify that the sampling distribution of can be approximated by the normal distribution.

4. Find the critical value zc that corresponds to the given level of confidence c.

ˆ xpn

Use the Standard Normal Table

.̂p

5, 5ˆ ˆnp nq p̂

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In Words In Symbols

Constructing Confidence Intervals for p

5. Find the margin of error E.

6. Find the left and right endpoints and form the confidence interval.

ˆ ˆc

pqE zn

Left endpoint: Right endpoint: Interval:

p̂ Ep̂ E

ˆ ˆp E p p E

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In Words In Symbols

Example: Confidence Interval for p

In a survey of 1219 U.S. adults, 354 said that their favorite sport to watch is football. Construct a 95% confidence interval for the proportion of adults in the United States who say that their favorite sport to watch is football.

Solution: Recall ˆ 0.290402p

1 0.290402ˆ ˆ 0.7095981q p

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Solution: Confidence Interval for p

• Verify the sampling distribution of can be approximated by the normal distribution

p̂

1219 0.290402 354 5ˆnp

1219 0.709598 865 5ˆnq • Margin of error:

(0.290402) (0.709598)1.96ˆ ˆ 0.0251219c

pqE zn

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Solution: Confidence Interval for p

• Confidence interval:

ˆ

0.29 0.025

0.265

p E

Left Endpoint: Right Endpoint:

0.265 < p < 0.315

ˆ

0.29 0.025

0.315

p E

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Solution: Confidence Interval for p

• 0.265 < p < 0.315

( )• 0.290.265 0.315

With 95% confidence, you can say that the proportion of adults who say football is their favorite sport is between 26.5% and 31.5%.

Point estimate

p̂p̂ E p̂ E

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Sample Size

• Given a c-confidence level and a margin of error E, the minimum sample size n needed to estimate p is

• This formula assumes you have an estimate for and .

• If not, use and

2

ˆ ˆ czn pq

E

ˆ 0.5.qˆ 0.5p

p̂q̂

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Example: Sample Size

You are running a political campaign and wish to estimate, with 95% confidence, the proportion of registered voters who will vote for your candidate. Your estimate must be accurate within 3% of the true population. Find the minimum sample size needed if

1.no preliminary estimate is available.

Solution: Because you do not have a preliminary estimate for use and ˆ 5.0.q ˆ 0.5p p̂

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Solution: Sample Size

• c = 0.95 zc = 1.96 E = 0.03

2 21.96

(0.5)(0.5) 1067.110.

ˆ03

ˆ czn pq

E

Round up to the nearest whole number.

With no preliminary estimate, the minimum sample size should be at least 1068 voters.

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Example: Sample Size

You are running a political campaign and wish to estimate, with 95% confidence, the proportion of registered voters who will vote for your candidate. Your estimate must be accurate within 3% of the true population. Find the minimum sample size needed if

2.a preliminary estimate gives . ˆ 0.31p

Solution: Use the preliminary estimate

1 0.31 0. 9ˆ ˆ 61q p

ˆ 0.31p

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Solution: Sample Size

• c = 0.95 zc = 1.96 E = 0.03

2 21.96

(0.31)(0.69) 913.020.

ˆ ˆ03

czn pq

E

Round up to the nearest whole number.

With a preliminary estimate of , the minimum sample size should be at least 914 voters.Need a larger sample size if no preliminary estimate is available.

ˆ 0.31p

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• End, any questions?

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