Dependent t-Test

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# Dependent t-Test - PowerPoint PPT Presentation

Dependent t-Test. CJ 526 Statistical Analysis in Criminal Justice. Overview. Dependent Samples Repeated-Measures. When to Use a Dependent t-Test. Two Dependent Samples Repeated-Measures Design (before-after) Matched-Subjects Design. Example of a Dependent t-Test.

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## Dependent t-Test

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### Dependent t-Test

CJ 526 Statistical Analysis in Criminal Justice

Overview
• Dependent Samples
• Repeated-Measures
When to Use a Dependent t-Test
• Two Dependent Samples
• Repeated-Measures Design (before-after)
• Matched-Subjects Design
Example of a Dependent t-Test
• A forensic psychologist wants to determine whether physical exercise in a boot camp program has an effect on muscular strength. He/she measures the number of pull-ups 25 program participants complete at the beginning of the program (M = 3.28, SD = 1.88) and at the end of the program (M = 3.6, SD = 1.73).
Example of a Dependent t-Test -- continued
• Number of Samples: 2
• Nature of Samples: dependent (same subjects at two different points in time)
•  Known:
Example of a Dependent t-Test -- continued
• Independent Variable: participation in boot camp--exercise
• Dependent Variable and its Level of Measurement: number of pull-ups
• Target Population: boot camp participants
Example of a Dependent t-Test -- continued
• Appropriate Inferential Statistical Technique: t test, related samples
• Null Hypothesis: no difference between the groups before and after
• Alternative Hypothesis: there will be a difference, boot camp participants will be able to do more pullups after training
• Decision Rule:
• If the p-value of the obtained test statistic is less than .05, reject the null hypothesis, one-tail test
Example of a Dependent t-Test -- continued
• Obtained Test Statistic: t
• Decision: accept or reject null hypothesis
• D.f. = n-1

(in this case n – 1 = 25 – 1 = 24

Results Section
• The results of the Dependent t-Test involving participating in a physical exercise program as the independent variable and number of pull-ups as the dependent variable were not statistically significant, t (24) = -1.693.
Discussion Section
• It appears that participating in a physical exercise program does not have an effect on developing muscular strength among participants in a boot camp program.
SPSS Paired-Samples t-Test Procedure
• Analyze, Compare Means, Paired-Samples t-Test
• Move pair of variables over to Paired Variables
SPSS Paired-Samples t-Test Printout
• Paired Sample Statistics
• Paired variables
• Mean
• N
• Standard Deviation
• Standard Error of the Mean
SPSS Paired-Samples t-Test Printout -- continued
• Paired Samples Correlations
• Paired variables
• N
• Correlation
• Sig
• p-value of correlation coefficient
SPSS Paired-Samples t-Test Printout -- continued
• Paired Samples Test
• Paired variables
• Paired Differences
• Mean of the Difference
• Standard Deviation of the Difference
• Standard Error of the Mean of the Difference
• 95% Confidence Interval of the Difference
• Lower
• Upper
SPSS Paired-Samples t-Test Printout -- continued
• t: obtained test statistic
• df: degrees of freedom
• Sig: p-value
• Divide by 2 to get one-tailed p-value