title: Statistical problems caused by missing data resulting from neodymium:YAG laser capsulotomies in long-term posterior capsule opacification studies: problem identification and possible solutions.
abstract: OBJECTIVE
  To describe a way to eliminate the problem caused by missing data due to neodymium:YAG (Nd:YAG) laser capsulotomies before statistical analysis, which leads to incomplete data in long-term studies of posterior capsule opacification (PCO).
  
  
  METHODS
  Medical University of Vienna, Vienna, Austria.
  
  
  METHODS
  To demonstrate the problem, an existing long-term study comparing PCO development in eyes with round-edged and sharp-edged intraocular lenses (IOLs) was analyzed. In the study, several Nd:YAG capsulotomies led to dropouts in the round-edged IOL group. To solve the resulting missing value problem, the missing values were replaced by estimated PCO values in various scenarios; that is, different assumptions were made of how PCO would have developed without a capsulotomy. Standard statistical analyses that included the estimated PCO values rather than excluding the incomplete cases were then performed.
  
  
  RESULTS
  The analysis of all cases (including the estimated PCO scores) showed a highly significant difference between the round-edged group and the sharp-edged group. Sensitivity analyses confirmed that the difference between the 2 groups remained significant up to highly implausible values chosen for the missing PCO scores, thus proving the correctness of the analysis.
  
  
  CONCLUSIONS
  This study showed it is possible to perform statistical analyses without excluding incomplete cases. This approach is recommended for all studies with Nd:YAG dropouts as it increases the statistical power. Excluding incomplete cases may lead to biased or wrong study conclusions.
authors.affiliation: Department of Ophthalmology, Medical University of Vienna, Vienna, Austria.
authors.forename: Wolf
authors.initials: W
authors.lastname: Buehl
authors.forename: Harald
authors.initials: H
authors.lastname: Heinzl
authors.forename: Martina
authors.initials: M
authors.lastname: Mittlboeck
authors.forename: Oliver
authors.initials: O
authors.lastname: Findl
mesh_terms: D002386:Cataract / Q000209:etiology / Q000628:therapy; D003627:Data Interpretation, Statistical; D006801:Humans; D053685:Laser Therapy / Q000379:methods*; D007903:Lens Capsule, Crystalline / Q000473:pathology / Q000601:surgery*; D015233:Models, Statistical; D010352:Patient Dropouts / Q000706:statistics & numerical data; D011183:Postoperative Complications; D013223:Statistics as Topic / Q000379:methods*