study design parallel group design independent measure design Participants are assigned to one of two or more groups in parallel for the duration of the study http://www.holah.karoo.net/experimentaldesigns.htm Philippe Rocca-Serra PMID: 17408389-Purpose: Proliferative vitreoretinopathy (PVR) is the most important reason for blindness following retinal detachment. Presently, vitreous tamponades such as gas or silicone oil cannot contact the lower part of the retina. A heavier-than-water tamponade displaces the inflammatory and PVR-stimulating environment from the inferior area of the retina. The Heavy Silicone Oil versus Standard Silicone Oil Study (HSO Study) is designed to answer the question of whether a heavier-than-water tamponade improves the prognosis of eyes with PVR of the lower retina. Methods: The HSO Study is a multicentre, randomized, prospective controlled clinical trial comparing two endotamponades within a two-arm parallel group design. Patients with inferiorly and posteriorly located PVR are randomized to either heavy silicone oil or standard silicone oil as a tamponading agent. Three hundred and fifty consecutive patients are recruited per group. After intraoperative re-attachment, patients are randomized to either standard silicone oil (1000 cSt or 5000 cSt) or Densiron((R)) as a tamponading agent. The main endpoint criteria are complete retinal attachment at 12 months and change of visual acuity (VA) 12 months postoperatively compared with the preoperative VA. Secondary endpoints include complete retinal attachment before endotamponade removal, quality of life analysis and the number of retina affecting re-operation within 1 year of follow-up. Results: The design and early recruitment phase of the study are described. Conclusions: The results of this study will uncover whether or not heavy silicone oil improves the prognosis of eyes with PVR. parallel group design A parallel group design or independent measure design is a study design which uses unique experimental unit each experimental group, in other word no two individuals are shared between experimental groups, hence also known as parallel group design. Subjects of a treatment group receive a unique combination of independent variable values making up a treatment pending final vetting