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The Early Treatment for Retinopathy of Prematurity Clinical Trial: presentation by subgroups versus analysis within subgroups
  1. R J Hardy1,
  2. W V Good2,
  3. V Dobson3,
  4. E A Palmer4,
  5. B Tung5,
  6. D L Phelps6,
  7. M J Shapiro7,
  8. W A J van Heuven8,
  9. on behalf of the ETROP Cooperative Group
  1. 1University of Texas-Houston, School of Public Health, Houston, Texas, USA
  2. 2Smith-Kettlewell Eye Research Institute, San Francisco, California, USA
  3. 3Department of Ophthalmology and Visual Science, University of Arizona, Tucson, Arizona, USA
  4. 4Casey Eye Institute, Oregon Health and Science University, Portland, Oregon, USA
  5. 5University of Texas-Houston, School of Public Health, Houston, Texas, USA
  6. 6University of Rochester Medical Center, Rochester, New York, USA
  7. 7Department of Ophthalmology, University of Illinois at Chicago, Chicago, Illinois, USA
  8. 8University of Texas Health Science Center at San Antonio, San Antonio, Texas, USA
  1. Correspondence to: R J Hardy University of Texas-Houston, School of Public Health, 1200 Herman Pressler, Houston, TX 77030, USA; Robert.J.Hardy{at}

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An ETROP study in infants with retinal ablation of high-risk prethreshold ROP: subgroup results

The Early Treatment for Retinopathy of Prematurity (ETROP) Study showed that retinal ablation for high-risk prethreshold retinopathy of prematurity (ROP) improves structural and functional outcomes at 9 months’ corrected age, compared with conventional management.1 As reported in this issue of the British Journal of Ophthalmology, (see p 1378) the benefit for structural outcome extends at least until 2 years of age.2 To discern a significant benefit for earlier treatment, the study randomised 401 infants who had prethreshold disease, and who also had a risk for unfavourable structural outcome ⩾15%. Approximately 80% of the infants had high-risk disease in both eyes (one treated at prethreshold and the other managed conventionally). The other 20% of infants had one eye eligible, and that eye was randomised to early treatment at prethreshold or to conventional management. The fellow eye was managed conventionally. More details about risk calculation, patient selection and study design are presented elsewhere.1,3,4

Interest in subgroups in the study has occurred since the publication of the original report from the ETROP Study.1,5 The purpose of this communication is to discuss treatment decisions that might be based on subgroup findings. We delineate two distinct ways of viewing the results of data in clinical trials when the data are (1) presented by subgroups and (2) analysed within subgroups.


Virtually all clinical trials report data by giving the overall results for the primary hypothesis followed by tables that present the results in separate subgroups. The ETROP Study reported findings for various subgroups, including the International Classification for Retinopathy of Prematurity (ICROP) categories (eg, zone I, stage 1 …

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