Statistical problems caused by missing data resulting from neodymium:YAG laser capsulotomies in long-term posterior capsule opacification studies Problem identification and possible solutions Wolf Buehl, MD, Harald Heinzl, PhD, Martina Mittlboeck, PhD, Oliver Findl, MD PURPOSE: To describe a way to eliminate the problem caused by missing data due to neody- mium:YAG (Nd:YAG) laser capsulotomies before statistical analysis, which leads to incomplete data in long-term studies of posterior capsule opacification (PCO). SETTING: 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 signif- icant 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. J Cataract Refract Surg 2008; 34:268–273 Q 2008 ASCRS and ESCRS Posterior capsule opacification (PCO), or after- cataract, remains the most frequent long-term compli- cation of cataract surgery with intraocular lens (IOL) implantation. 1–3 Usually, PCO develops over years and leads to a slow decrease in visual acuity. As soon as the center of the posterior capsule (around the visual axis) is affected, visual acuity can be signif- icantly impaired. Therefore, many laboratory and clin- ical studies evaluate the pathogenesis and formation of PCO and possible ways to prevent it. To properly judge the potential of a new IOL or surgical method to prevent PCO, long-term follow-up (usually 3 years or longer) of the included eyes is necessary. Posterior capsule opacification can be treated by opening the posterior lens capsule by a neodymium: YAG laser (Nd:YAG) capsulotomy. Although Nd:YAG laser capsulotomy is an effective way of elim- inating the adverse effects of PCO, such cases usually drop out of follow-up in clinical trials of PCO because PCO assessment is not possible after the capsulotomy. These dropouts hinder statistical analysis of the data. For ethical reasons, it is usually not possible to post- pone the capsulotomy to after the end of the trial (eg, after the 3-year follow-up examination) because the decreased visual acuity is unacceptable and likely to worsen. In addition, excluding cases with missing Q 2008 ASCRS and ESCRS 0886-3350/08/$dsee front matter Published by Elsevier Inc. doi:10.1016/j.jcrs.2007.11.014 268 ARTICLE