Psychology week 2 assignment

Repeated Measures ANOVA Assignment

This assignment is based on a study by Foa, Rothbaum, Riggs, and Murdock (1991) published in the Journal of Counseling and Clinical Psychology. The study investigates PTSD symptom severity among 45 rape victims randomly assigned to one of four groups:

· Stress Inoculation Therapy (SIT): Subjects were taught various coping skills (similar to cognitive therapy)

· Prolonged Exposure (PE): Subjects revisited the traumatic event mentally during seven sessions (based on exposure therapy)

· Supportive Counseling (SC): Subjects received attention and positive regard but no specific therapy

· Waiting List (WL) Control: No therapy, just measurement

For simplicity, we will focus on PTSD Severity, defined as the total number of PTSD symptoms reported by participants, measured at three points:

· Pre-treatment

· Post-treatment

· One-month follow-up

Study Design

This study included 45 rape victims randomly assigned across the four treatment conditions.

Summary of Variables

Independent Variables:

· Between-Subjects Variable: Therapy Type (4 conditions: SIT, PE, SC, WL)

· Within-Subjects Variable: Time (3 levels: pre, post, follow-up)

Dependent Variable:

· PTSD Severity (the sum of the subject's ratings on about 15 variables related to PTSD—e.g., flashbacks, nightmares, memory difficulties, etc.)

Assignment Instructions

When preparing your assignment for submission, be sure to keep the questions in and write your answers below each question.

Part 1: Overall Effect of Time

1. Before conducting the repeated-measures ANOVA, examine the data for outliers:

· Create boxplots for PTSD severity at each time point

· Identify any extreme outliers in the data

· If outliers are present, discuss their potential impact on the analysis

Q1a) Were any significant outliers detected in the PTSD severity data? If so, provide details about their values and positions (e.g., which time point they occurred at). Would you keep, remove, or transform these data points for further analysis? Justify your decision. Q1b) Although we could use multiple paired sample t-tests to analyze changes between different time points (pre-post, post-follow-up, and pre-follow-up), this approach has limitations. What is one significant problem with using multiple t-tests?

2. Conduct a one-way repeated measures ANOVA to examine the overall effect of time on PTSD severity (combining all treatment groups). Be sure to:

· Check the assumption of sphericity using Mauchly's test

· If sphericity is violated, use appropriate corrections (Greenhouse-Geisser or Huynh-Feldt)

· Note: While multivariate test results (Pillai's Trace, Wilks' Lambda, etc.) will be available, we will focus on the univariate results for this analysis.

Q2) What is the null hypothesis (H0) for this test? Q3) Based on your analysis, can we reject the null hypothesis and conclude there was a significant change over time? Include the F-test results and p-value in your answer. Q4) Examine the profile plot showing the change in means over time. What does it suggest about the change in PTSD symptoms over time? Include the profile plot (labeled per APA guidelines) and interpret the results of the F-test.

Part 2: Effect of Therapy Type

Conduct a mixed ANOVA (also called split-plot ANOVA) to examine both the effect of time and the interaction between time and therapy type.

Q5) Was the interaction between time and group significant (i.e., do the groups differ from each other over time)? Include the interaction F-test result and p-value. Additionally, provide the interaction profile plot (labeled per APA guidelines) to support your answer.

Q6) Using the profile plot, describe how the treatment effects differed across time. What trends do you observe for each therapy group?

Part 3: Results Write-Up

Q7) Write a formal results section as if for a manuscript. Include:

· An introduction to the analysis

· Descriptive statistics for PTSD severity (means and standard deviations for each time point and therapy group)

· A report of the main effects (time, group) and the interaction effect (time by group), using proper APA statistical notation

· Tables and/or figures to summarize the data and results (ensure they adhere to APA style)

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