1. Experimental Clinical Psychology Session V
Eiko Fried
Department of Clinical Psychology
Leiden University

2. Q1 Remission vs transition
Q1: The abstract of Van der Gaag et al. (2012) states, in the first sentence of “Results”, that the “number needed to treat” (NNT) was 9. Show how this number was calculated and explain what it means.

3. Q1

4. Q1
Is the NNT rounded? If so, how and why?

5. Q2
Of the patients who were randomized to the experimental intervention condition (CBTuhr), “16 patients had no sessions at all; 21 had 1–5 sessions; 16 had 6–11 sessions; and 45 had 12–25 sessions.” (p. 1182). The authors could have chosen to (additionally) present analyses for those patients who actually completed a minimum number of sessions. Provide one argument to do this, and one argument to support the authors’ choice NOT to do this.

6. Q2 For: Gives some more clarity on mechanism of action
Otherwise, effect underestimated (“reverse” diffusion of treatment, or rather diffusion of control condition)
Against:
Realistic setting
Randomization
Arbitary cut-off
Power

7. Q3
Van der Gaag et al. (2012) discovered during the study that 5 patients had violated exclusion criteria (page 1183). All of these 5 patients transitioned into psychosis. The authors present two sets of analyses of the primary outcome measure: a regular intention-to-treat analysis from which these patients were excluded, and an intention-to-treat analysis including these 5 patients. a) What were the results for these two analyses, for the primary outcome measure, and what is the main difference between these two results?

8. Q3

9. Q3
Difference between p=0.03 and p=0.06 is usually not significant itself

10. Q3
Difference between p=0.03 and p=0.06 is usually not significant itself

11. Q3
b) Which of these two results, mentioned in question 3a, do you think is the “correct” one, and therefore the main result of this study? In other words, do you think it is correct to exclude these 5 patients, or should they be included? Please explain
They meet exclusion criteria …

12. Q4
According to Figure 1, 302 eligible patients remained, 201 of whom eventually agreed to participate and were randomized. The reasons why the other 101 participants were not randomized are described as: “57 refused to participate, 22 no contact, 22 other reasons”. Unfortunately, the authors did not present a non-response analysis so we don’t know much about these 101 persons.

13. Q4
a) Do you think the self-selection of participants could form a threat to the external validity (generalizability) of the results of the study? Please explain.
b) Do you think the self-selection of participants and / or the selection by the researchers (inclusion criteria) could form a threat to the internal validity of the results of the study? Explain your answer.

14. Q5

15. Q5
Cockup before conspiracy

16. Additional topics
“The scores of all therapists were (at least) at competent level (17.5% was competent; 55% proficient; and 27.5% expert level).”
DSM-5 field trials:

17. Methodological strenghts
Mention important methodological strengths of the study by Van der Gaag et al. (2012), and briefly explain why they are strengths:
Treatment fidelity
Trained and supervised therapists (less variability in treatments)
Blinded raters and replacement of raters when blinding was broken
Multiple centres/therapists
Long follow-up (even though the authors call it a short follow-up)
RCT

19. So far …
Mean % correct from assignments:
84.3%
81.8%
84.7%
76.1%

20. Student wants to join prep group

21. And now: this

23. Break