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Dependent t-Test

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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  1. Dependent t-Test CJ 526 Statistical Analysis in Criminal Justice

  2. Overview • Dependent Samples • Repeated-Measures

  3. When to Use a Dependent t-Test • Two Dependent Samples • Repeated-Measures Design (before-after) • Matched-Subjects Design

  4. 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).

  5. Example of a Dependent t-Test -- continued • Number of Samples: 2 • Nature of Samples: dependent (same subjects at two different points in time) •  Known:

  6. 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

  7. 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

  8. 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

  9. 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.

  10. 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.

  11. SPSS Paired-Samples t-Test Procedure • Analyze, Compare Means, Paired-Samples t-Test • Move pair of variables over to Paired Variables

  12. SPSS Paired-Samples t-Test Sample Printout

  13. SPSS Paired-Samples t-Test Printout • Paired Sample Statistics • Paired variables • Mean • N • Standard Deviation • Standard Error of the Mean

  14. SPSS Paired-Samples t-Test Printout -- continued • Paired Samples Correlations • Paired variables • N • Correlation • Sig • p-value of correlation coefficient

  15. 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

  16. 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

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