Question 1. [6 marks]

A psychologist compared the effectiveness of Cognitive Behavioural Therapy (CBT) and eye movement desensitisation and reprocessing (EMDR) for the treatment of post-traumatic stress disorder (PTSD). Clients with PTSD were recruited and randomised into either the CBT or EMDR group. A group of 5 psychologists were trained in both treatment methods. Individual clients were treated by the same psychologist across the study. The Clinician-Administered PTSD Scale (CAPS) was used to assess severity of PTSD symptoms before and after the course of therapy.

They then analysed the difference in CAP Scale. The predictor variables were psychologist, and treatment method.

1. What analysis should be used for these data? Explain why. Write the statistical model for this as an equation.
2. Could one of the predictor variables be thought of as a block variable in this case? Why?

Question 2. [14 marks]

The file PsychBike.sav contains the results of research investigating drivers' behaviour towards cyclists. The experiment involved observing the way in which the motor vehicle passed a cyclist at different times of the day. The researchers varied the distance the cyclist was riding from the kerb, and whether they were wearing a helmet. The response variable of interest was the passing distance, the distance between the cyclist and the motor vehicle at the time the motor vehicle passed the bicycle.

The variables in the file are:

• vehicle : Vehicle type
• Colour : Vehicle colour
• hour : hour of observation
• helmet : was the cyclist wearing a helmet? 0 = `no', 1=`yes'
• distance from kerb : distance between the cyclist and the kerb
• passing distance : distance between the cyclist and the motor vehicle

(a) Is this an experiment or an observational study? Why?

(b) Carry out a two factor ANOVA to investigate the effect of helmet wearing and distance from kerb (treated as a categorical variable) on passing distance. Be sure to include a whole model test as well as tests of the factors and interaction.

(c) Are the assumptions of ANOVA met in this case?

(d) Try the log transformation of passing distance and the square root transformation o passing distance. Does either of these improve the extent of meeting assumptions?

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Solution: The downloadable solution consists of 5 pages, 749 words.
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