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

When to Use a Dependent t-Test Two Dependent (related) 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 (before & after) Nature of Samples: dependent (same subjects at two different points in time)

Example of a Dependent t-Test -- continued 3. Independent Variable: participation in boot camp--exercise 4. Dependent Variable and its Level of Measurement: number of pull-ups (ratio) 5. Target Population: boot camp participants

Example of a Dependent t-Test -- continued 6. Appropriate Inferential Statistical Technique: t test, related samples Null Hypothesis: no difference between the groups before and after boot camp exercise Alternative Hypothesis: there will be a difference, boot camp participants will be able to do more pull ups 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 10. 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.

Discussion Section It appears that participating in a physical exercise program did 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 Sample Printout

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