Hmm, I would actually love to know the answer to your question. I am not aware of any recent meta-analyses that looked into this important question. The question of drop-out is an important issue to look into thoroughly: There are those who drop out because they don't like the treatment, or because they find it too stressful, or because they deteriorate, but also because they get better very quickly, so that they feel they no longer need treatment.

Have you checked the Veterans Affair Magazine archives for articles on this topic?

http://www.va.gov/opa/publications/archives/arch_vanguard.asp

Drop-out is an important factor in any treatment. Don't forget to consider the most-researched of all the recent non-traditional treatments: Transcendental Meditation. TM has been used to treat PTSD for several years (in individual cases as well as in organized studies), and the TM organization itself is probably a good source of research sets, or at least of statistical summaries, not only of drop-out rates, but of other statistics relevant to PTSD as well.

In case you weren't aware, the USA Veterans Administration is currently running at least one study to evaluate TM as a treatment for PTSD. TM is attractive because it eliminates stress and anxiety, normalizes various aspects of the nervous system, reduces or eliminates the symptoms of PTSD, and produces few or no unwanted side effects. Other and cheaper means of teaching the technique of transcending, such as Natural Stress Relief (NSR) are also available to the public, although these alternative techniques have not been studied as treatments for PTSD or other specific disorders.

Websites for further information: TM: www.tm.org ; NSR: www.nsr.org .

Thanks all for your helpful comments.  I'll follow-up on your suggestions.  A review (or meta-analysis) comparing the drop-out rates of various treatments for PTSD would be interesting.  On a side note, a colleague of mine recently directed me to an article that I found quite interesting regarding comparison of two VA facilities and responses to EMDR.  Made for some interesting reading:

Cook, J.M., Biyanova, T., & Coyne, J.C. (2009).  Comparative case study of diffusion of eye movement desensitization and reprocessing in two clinical settings: Empirically supported treatment status is not enough.  Professional Psychology: Research and Practice, 40(5), 518-524.  doi: 10.1037/a0015144.        

Glen Palmer, What was the result of the EMDR study? How many subjects were there?

It was not a quantitative study, but rather, a qualitative study.  However, the authors conducted semistructured interviews of mental health professionals from a VA facility that had adopted EMDR (n = 10) versus another facility that did not adopt EMDR (n = 19) for treatment.  What was interesting to me were responses from the site did not use EMDR.  Despite acknowledging the evidence base and empical support for the treatment, there was resistance from clinicians due to "not understanding the mechanism of action" and "not being compatible with their understanding of psychotherapy."  Unfortunately, I still occasionally encounter clinicians who are unaware that EMDR is an empirically supported treatment for PTSD.              

Try some of these.  The foiirst four came up on a google search.

Van Minnen, A., Hamed, M., Zoelner, L, & Mills, K. (2015). Examining potential contraindications for prolonged exposure therapy for PTSD. European Journal of Psychotraumatology 2012, 3: 18805 - http://dx.doi.org/10.3402/ejpt.v3i0.18805

Imel, Z. E., Laska, K, Jakupcak, M., & Simpson, T.L.  (2013). Meta-Analysis of Dropout in Treatments for Posttraumatic Stress Disorder. Journal of Consulting and Clinical Psychology In the public domain; 81(3): 394–404

Shottenbauer, M. A., Glass, R. G., Arnkoff, D. B., et al. (2008). Nonresponse and Dropout Rates in Outcome Studies on PTSD: Review and Methodological Considerations. Psychiatry 71(2) Summer 2008

Najavits, L. M. (2015). The problem of dropout from “gold standard” PTSD therapies. F1000Prime Reports 2015, 7:43 (doi:10.12703/P7-43).

Albright, D., Becker, B., Rubin, A., & Thyer, B. A. (2011). Eye movement desensitization and reprocessing (EMDR) for posttraumatic stress disorder (PTSD) in combat veterans. Campbell Systematic Reviews, 2011:7.

Barrera, T. L., Mott, J. M., Hofstein, R. F., & Teng, E. J. (2013). A meta-analytic review of exposure in group cognitive behavioral therapy for posttraumatic stress disorder. Clinical Psychology Review, 33(1), 24-32. doi: http://dx.doi.org/10.1016/j.cpr.2012.09.005

Bisson, J., & Andrew, M. (2007). Psychological treatment of post-traumatic stress disorder (PTSD). Cochrane Database Syst Rev (3), CD003388. doi: 10.1002/14651858.CD003388.pub3.

Goodson, J., Helstrom, A., Halpern, J. M., Ferenschak, M. P., Gillihan, S. J., & Powers, M. B. (2011). The treatment of posttraumatic stress disorder in U.S. combat veterans: A meta-analytic review. Psychological Reports, 109(2), 573-599. doi: 10.2466/02.09.15.16.PR0.109.5.573-599.

Monson, C., Gradus, J., Young-Xu, Y., Schnurr, P., Price, J., & Schumm, J. A. (2008). Change in posttraumatic stress disorder symptoms: Do clinicians and patients agree? Psychological Assessment, 20(2), 131-138.