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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.
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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
