Hypothesis Testing Introduction to

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Introduction to

Hypothesis Testing

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Section Goals

After completing this section, you should be able to:

 Formulate null and alternative hypotheses for

applications involving a single population mean or proportion

 Formulate a decision rule for testing a hypothesis

 Know how to use the test statistic, critical value, and p-value approaches to test the null hypothesis

 Know what Type I and Type II errors are

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Chap 8-3

What is a Hypothesis?



A hypothesis is a claim (assumption) about a population parameter:

Example you friend makes the following claims:

 population mean

 population proportion

The mean monthly cell phone bill of this city is  = $42

The proportion of adults in this city with cell phones is p = .68

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The Null Hypothesis, H

₀



A statement about a population parameter that will be assumed to be true during the conduct of the hypothesis test.



The null hypothesis is either rejected or accepted based on a sample data

(continued)

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The Null Hypothesis, H

₀

μ 3 :

H

₀



^H₀ ^: ^x  ³

 States the assumption (numerical) to be tested Example: The average number of TV sets in a

typical Ghanaian home is at least 3

The test is always about the population

parameter, not about a sample statistic, although the test is proven using a sample.

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The Null Hypothesis, H

₀



Begin with the assumption that the null hypothesis is true



Similar to the notion of innocent until proven guilty



Refers to the status quo



Always contains “=” , “≤” or “” sign



May or may not be rejected

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The Alternative Hypothesis, H

_A

 Is the opposite of the null hypothesis

 e.g.: The average number of TV sets in

Ghanaian homes is less than 3 ( H_A:  < 3 )

 Challenges the status quo

 Never contains the

“=” , “≤” or “”

^sign

 May or may not be accepted

 Is generally the hypothesis that is believed (or needs to be supported) by the

researcher

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Formulating a hypothesis



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Chap 8-9

Formulating a hypothesis



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Errors in Making Decisions

 Different samples would give different values, therefore result based on samples could

technically give results quite different from the population mean.

 For example, the null hypothesis about the

population could be true but due to using result from a sample, it could be rejected.

 Likewise, the alternative hypothesis could be rejected although true.

 In such cases, we say, the researcher has made

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Errors in Making Decisions

 Type I Error

 Reject a true null hypothesis

 Considered a serious type of error The probability of Type I Error is



 Called level of significance of the test

 Set by researcher in advance

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Errors in Making Decisions

 Type II Error

 Fail to reject a false null hypothesis The probability of Type II Error is β

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Outcomes and Probabilities

State of Nature Decision

Do Not Reject

H ₀

No error

(1 - )  Type II Error ( β )

Reject H ₀

Type I Error ( ) 

Possible Hypothesis Test Outcomes

H₀ False H₀ True

Key:

Outcome

(Probability) No Error

( 1 - β )

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Level of Significance, 

 The level of significance is the probability of making a Type I error when the null hypothesis is true as

an equality.

 Defines rejection region of the sampling distribution

 Is designated by



, (level of significance)

 Typical values are .01, .05, or .10

 Is selected by the researcher at the beginning

 Provides the critical value(s) of the test

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Level of Significance, 



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Level of Significance and the Rejection Region

H₀: μ ≥ 3 H_A: μ < 3

0

H₀: ^μ ≤ 3 H_A: μ > 3 H₀: ^μ = 3 H : μ ≠ 3



/2

Represents critical value

Lower tail test

Level of significance =



0

/2



Upper tail test

Rejection region is shaded

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Critical Value

Approach to Testing

 Convert sample statistic (e.g.: ) to test statistic ( Z or t statistic )

 Determine the critical value(s) for a specified level of significance  from a table or

computer

 If the test statistic falls in the rejection region, reject H₀ ; otherwise do not reject H₀

x

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Reject H₀ Do not reject H₀

 The cutoff value -z_α, is called a critical value

One Tailed Test: Lower Tail



-z_α ₀

H₀: μ ≥ 3 H_A: μ < 3

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

z_α

0

H₀: μ ≤ 3 H_A: μ > 3

One Tailed Test: Upper Tail

 The cutoff value z_α, is called a critical value

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Do not reject H₀ Reject H₀ Reject H₀

 There are two cutoff values (critical values):

Two Tailed Tests

/2

-z_α/2

± z_α/2

0

H₀: μ = 3 H_A: μ  3

z_α/2

/2

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Critical Value

Approach to Testing

 Convert sample statistic ( ) to a test statistic ( Z or t statistic )

x

 Known

Large Samples

 Unknown Hypothesis

Tests for 

Small Samples

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 Known

Tests for μ

Small Samples The test statistic is:

Calculating the Test Statistic

σ μ z x 



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 Known

Tests for 

Calculating the Test Statistic

n s

μ t

_n_₁

 x 

But is sometimes approximated using a z:

n σ

μ z x 



(continued)

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 Known

Tests for 

Calculating the Test Statistic

n s

μ t

_n_₁

 x 

(The population must be

(continued)

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Review: Steps in Hypothesis Testing

 1. Specify the population value of interest

 2. Formulate the appropriate null and alternative hypotheses

 3. Specify the desired level of significance

 4. Determine the rejection region

 5. Obtain sample evidence and compute the test statistic

 6. Reach a decision and interpret the result

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Hypothesis Testing Example

Test the claim that the true mean # of TV sets in

Ghanaian homes is at least 3. Assume σ = 0.8 and that a sample of 100 is taken and the sample mean is 2.84)

 1. Specify the population value of interest

 The mean number of TVs in Ghanaian homes

 2. Formulate the appropriate null and alternative hypotheses

 H₀: μ  3 H_A: μ < 3 (This is a lower tail test)

 3. Specify the desired level of significance Suppose that  = .05 is chosen for this test

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 4. Determine the rejection region

 = .05

-z_α= -1.645 ₀

This is a one-tailed test with  = .05.

Since σ is known, the cutoff value is a z value:

Reject H₀ if z < -z_ = -1.645 ; otherwise do not reject H₀

Hypothesis Testing Example

(continued)

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 5. Obtain sample evidence and compute the test statistic

n = 100, x = 2.84 ( = 0.8 is assumed known)

.08 2.0 .16 100

0.8 3 2.84

n σ

μ

z x   

 



Hypothesis Testing Example

Then the test statistic is:

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 = .05

-1.645 0

 6. Reach a decision and interpret the result

-2.0

Since z = -2.0 < -1.645, we reject the null hypothesis that the mean number of TVs in Ghanaian homes is at least 3

Hypothesis Testing Example

(continued)

z

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p-Value Approach to Testing

 Convert Sample Statistic (e.g. ) to Test Statistic ( Z or t statistic )

 Obtain the p-value from a table or computer

 Compare the p-value with 

 If p-value <  , reject H₀

 If p-value   , do not reject H₀ x

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p-Value Approach to Testing



p-value: Probability of obtaining a test statistic more extreme ( ≤ or  ) than the observed sample value given H

₀

is true

 Also called observed level of significance

 Smallest value of  for which H₀ can be rejected

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p-value =.0228

 = .05

p-value example

 Example: How likely is it to see a sample mean of 2.84 (or something further below the mean) if the true mean is  = 3.0?

2.8684 3 2.84

x .0228

2.0) P(z

100 0.8

3.0 z 2.84

P

μ 3.0)

| 2.84 x

P(























  





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 Compare the p-value with 

 If p-value <  , reject H₀

 If p-value   , do not reject H₀

Here: p-value = .0228  = .05

Since .0228 < .05, we reject the null hypothesis

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p-value example

p-value =.0228

 = .05

2.8684 3 2.84

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Example: Upper Tail z Test for Mean ( Known)

A phone industry manager thinks that customer monthly cell phone bill have increased, and now average over $52 per month. The company wishes to test this claim.

(Assume  = 10 is known and that a sample of 64 is taken and the sample mean is 53.1)

H₀: μ ≤ 52 the average is not over $52 per month

H_A: μ > 52 the average is greater than $52 per month

(i.e., sufficient evidence exists to support the

Form hypothesis test:

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 Suppose that  = .10 is chosen for this test Find the rejection region:

 = .10

z_α=1.28

0

Reject H₀

Reject H₀ if z > 1.28

Example: Find Rejection Region

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Obtain sample evidence and compute the test statistic

Suppose a sample is taken with the following

results: n = 64, x = 53.1 (=10 was assumed known)

 Then the test statistic is:

10 0.88 52 53.1

σ μ

z x  

 



Example: Test Statistic

(continued)

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Example: Decision

 = .10

1.28

0

Reject H₀

Do not reject H₀ since z = 0.88 ≤ 1.28

i.e.: there is not sufficient evidence that the mean bill is over $52

z = .88

Reach a decision and interpret the result:

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Reject H₀

 = .10

Do not reject H₀

1.28

0

Reject H₀

z = .88

Calculate the p-value and compare to 

(continued)

.1894

.3106 .5

0.88) P(z

64 10

52.0 z 53.1

P

52.0) μ

| 53.1 x

P(

























 





 p-value = .1894

p -Value Solution

since p-value = .1894 >  = .10

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Example: Two-Tail Test ( Unknown)

The average cost of a hotel room in New York is said to be $168 per

night. A random sample of 25 hotels resulted in x = $172.50 and

s = $15.40. Test at the  = 0.05 level.

(Assume the population distribution is normal)

H₀: μ = 168 H_A: μ  168

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  = 0.05

 n = 25

  is unknown, so use a t statistic

 Critical Value:

t = ± 2.0639

Example Solution: Two-Tail Test

Do not reject H : not sufficient evidence that

Reject H₀ Reject H₀

/2=.025

-t_α/2

Do not reject H₀

0 t_α/2

/2=.025

-2.0639 2.0639

1.46 25

15.40

168 172.50

n s

μ

t_n_₁  x     1.46

H₀: μ = 168 H_A: μ  168

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Hypothesis Tests for Proportions

 Involves categorical values

 Two possible outcomes

 “Success” (possesses a certain characteristic)

 “Failure” (does not possesses that characteristic)

 Fraction or proportion of population in the

“success” category is denoted by p

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Proportions

 Sample proportion in the success category is denoted by p



 When both np and n(1-p) are at least 5, p can be approximated by a normal distribution with mean and standard deviation



(continued)

size sample

sample in

successes of

number n

p  x 

μ  p

_σ _ ^p(1^ ^p)

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 The sampling

distribution of p is normal, so the test statistic is a z

value:

Hypothesis Tests for Proportions

n

) p (

p

p z p



 

1

np  5 and n(1-p)  5

Hypothesis Tests for p

np < 5 or

n(1-p) < 5

Not discussed in this chapter

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Example: z Test for Proportion

A marketing company claims that it receives 8% responses from its mailing. To test this

claim, a random sample of 500 were surveyed with 25 responses. Test at the ^{= .05}

significance level.

Check:

np = (500)(.08) = 40

n(1-p) = (500)(.92) = 460



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Z Test for Proportion: Solution

 = .05

n = 500, p = .05

Reject H₀ at  = .05

H₀: p = .08 H_A: p  .08

Critical Values: ± 1.96

Test Statistic:

Decision:

Conclusion:

0 z

Reject Reject

.025 .025

1.96

-2.47

There is sufficient

evidence to reject the company‟s claim of 8%

response rate.

2.47 500

.08) .08(1

.08 .05

n

p) p(1

p

z p  



 



 

-1.96

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Do not reject H₀

Reject H₀ Reject H₀

/2 = .025

1.96

0

z = -2.47

Calculate the p-value and compare to 

(For a two sided test the p-value is always two sided)

(continued)

0.0136 2(.0068)

.4932) 2(.5

2.47) P(x

2.47) P(z















p-value = .0136:

p -Value Solution

z = 2.47 -1.96

/2 = .025

.0068 .0068

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Section Summary



Addressed hypothesis testing methodology



Performed z Test for the mean (σ known)



Discussed p–value approach to hypothesis testing



Performed one-tail and two-tail tests . . .

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Section Summary



Performed t test for the mean (σ unknown)



Performed z test for the proportion

(continued)