Bourque LB, Lynn MJ, Waring GO 3rd, Cartwright C Spectacle and contact lens wearing six years after radial keratotomy in the Prospective Evaluation of Radial Keratotomy Study. Ophthalmology. 1994 Mar;101(3):421-31.
Cartwright CS, Bourque LB, Lynn M, Waring GO 3rd Relationship of glare to uncorrected visual acuity and cycloplegic refraction 1 year after radial keratotomy in the prospective evaluation of radial keratotomy (PERK) study. J Am Optom Assoc. 1988 Jan;59(1):36-9.
Lynn MJ, Waring GO 3rd, Carter JT Combining refractive error and uncorrected visual acuity to assess the effectiveness of refractive corneal surgery. Refract Corneal Surg. 1990 Mar-Apr;6(2):103-9; discussion 109-12.
Lynn MJ, Waring GO 3rd, Sperduto RD Factors affecting outcome and predictability of radial keratotomy in the PERK Study. Arch Ophthalmol. 1987 Jan;105(1):42-51.
McDonnell PJ, Nizam A, Lynn MJ, Waring GO 3rd Morning-to-evening change in refraction, corneal curvature, and visual acuity 11 years after radial keratotomy in the prospective evaluation of radial keratotomy study. The PERK Study Group. Ophthalmology. 1996 Feb;103(2):233-9.
McDonnell PJ, Schanzlin DJ Early changes in refractive error following radial keratotomy. Arch Ophthalmol. 1988 Feb;106(2):212-4.
Steinberg EB, Waring GO 3rd Comparison of two methods of marking the visual axis on the cornea during radial keratotomy. Am J Ophthalmol. 1983 Nov;96(5):605-8.
Waring GO 3rd, Lynn MJ, McDonnell PJ Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery. Arch Ophthalmol. 1994 Oct;112(10):1298-308.
Interventional studies are often prospective and are specifically tailored to evaluate direct impacts of treatment or preventive measures on disease.
Observational studies are often retrospective and are used to assess potential causation in exposure-outcome relationships and therefore influence preventive methods.
Expanded access is a means by which manufacturers make investigational new drugs available, under certain circumstances, to treat a patient(s) with a serious disease or condition who cannot participate in a controlled clinical trial.
Clinical trials are conducted in a series of steps, called phases - each phase is designed to answer a separate research question.
Phase 1: Researchers test a new drug or treatment in a small group of people for the first time to evaluate its safety, determine a safe dosage range, and identify side effects.
Phase 2: The drug or treatment is given to a larger group of people to see if it is effective and to further evaluate its safety.
Phase 3: The drug or treatment is given to large groups of people to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect information that will allow the drug or treatment to be used safely.
Phase 4: Studies are done after the drug or treatment has been marketed to gather information on the drug's effect in various populations and any side effects associated with long-term use.
