independent t test1

8/6/2019 Independent t Test1

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Independent t-Test

CJ 526 Statistical Analysis inCriminal Justice


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When to Use an Independent t-Test

1. Two samples

2. Interval or ratio level dependent variableEither

Experimental and control group comparison

OrComparing two separate independent groups

(no overlap)


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Characteristics of an Independent t-

Test

1. Sample means are hypothesized to be the

same. Either

1. Treatment has no effect, comparing an

experimental that received treatment and a

control group that did not receive treatment

2. Or, two independent groups are the same with

respect to a DV


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Example of an Independent t-Test --

continued

1. Number of Groups: 2

2. Nature of Groups: independent

3. Independent Variable: training

4. Dependent variable: number of

complaints


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continued

5. Dependent Variable and its Level ofMeasurement: complaints--interval

6. Target Population: employees

7. Appropriate Inferential StatisticalTechnique: t-test

8. One or two-tailed? Probably one tail


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continued

9. Null Hypothesis:

1. Mean of exp group mean of control group = 0

10. Alternative Hypothesis:Mean of experimental group minus mean of control

group does not equal 0

11. Decision Rule:1. If the p-value of the obtained test statistic is less

than .05, reject the null hypothesis


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continued

12. Obtained Test Statistic: t

13. Decision: accept or reject null hypothesis

Null hypothesistraining did not affect complaints,

comparing experimental and control groups

Alternative, one tailtraining reduced complaints ascompared to a control group without training

See p. 725


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Results Section

The results of the Independent t-Test using

diversity training as the independentvariable and number of complaints filed

against employees were statistically

significant, t (18) = 2.35, p < .05.

D.f. degrees of freedom = n(group

1)+n(group 2) - 2


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Discussion Section

It appears that employees undergoing

diversity trainingh

ave fewer complaintsfiled against them.

Or, if the null hypothesis was retained, the

conclusion would be that diversity training

did not affect the number of complaints

filed


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SPSS Independent-Samples t-

Test Procedure

Analyze, Compare Means, Independent-

Samples t-Test Move DV over to Test Variables

Move IV over to Grouping Variable

Enter numerical values of the IV underDefine Groups


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SPSS Independent-Samples t-Test

Sample Printout

T-Test

Group Statistics

10 23.80 14.816 4.685

10 28.70 14.833 4.691

Gender of Respondent

Female

Male

Score on Drink Index

N Mean Std. Deviation

Std. Error

Mean

Inde

endent

m

les Test

.086 .773 -.739 18 .469 -4.90 6.630 -18.828 9.028

-.739 18.000 .469 -4.90 6.630 -18.828 9.028

Equal variances

assumed

Equal variances

not assumed

Score on Drink Index

F Sig.

Levene's Test forEquality of Variances

t df Sig. (2-tailed)

Mean

Difference

Std. Error

Difference Lower Upper

95% Confidence

Interval of the

Difference

t-test for Equality of Means


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Test Printout

Group Statistics

DV Levels of IV

N: Sample size

Mean

Standard Deviation

Standard Error of the Mean


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Test Printout -- continued

Levenes Test for Equality of Variances

Test forh

omogeneity of variance assumption t-Test for Equality of Means

If Levene test is not significant

Equal variances assumed

If Levene test is significant

Equal variances not assumed


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t-Test for Equality of Means

t: obtained test statistic df: degrees of freedom

Sig: p-value

Divide by 2 to get one-tailed p-value

Mean Difference

Difference between the two sample means


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Standard Error of the Difference

95% Confidence Interval of the Difference Lower

Upper

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