A randomized clinical trial of 268 active-duty personnel seeking treatment for PTSD has found that individual sessions of cognitive processing therapy were twice as effective as group sessions.
Participants were assigned to CPT in either 90-minute group sessions or 60-minute individual sessions twice weekly for six weeks. PTSD severity was reduced significantly more in those receiving individual treatment, however improvements were maintained during the six-month follow-up in both groups. Depression and suicidal ideation also improved equally with both forms of CPT.
However, even among those receiving individual CPT, some 50% still had PTSD and clinically significant symptoms.
Full paper available at https://jamanetwork.com/journals/jamapsychiatry/fullarticle/2587076
 Effect of Group vs Individual Cognitive Processing Therapy in Active-Duty Military Seeking Treatment for Posttraumatic Stress Disorder: A Randomized Clinical Trial. JAMA Psychiatry [Internet]. 2017 ;74(1):28. Available from: http://archpsyc.jamanetwork.com/article.aspx?doi=10.1001/jamapsychiatry.2016.2729