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Hi All, One of the staff members on ATTRACT (www.attract.wales.nhs.uk) has asked my advice (see below) and I'm not able to help. So, I'm hoping that someone out there can help. Best wishes jon We have been asked what are the NNT and NNH for patients undergoing laser refractive eye surgery. There are three different types PRK, LASIK and LASEK NICE Interventional Procure Guidance on photorefractive (laser) surgery for the correction of refractive errors (1), published in 2006, has a sectionon efficacy; this states: In seven randomised controlled trials (RCTs) included in the review, there were no significant differences between the three procedures in the proportion of eyes treated for myopia or myopic astigmatism achieving the predicted refractive outcome. Data from more than 2000 eyes treated with PRK for myopia showed that a median of 69% of eyes had achieved within 0.5 D of their intended correction, and that 89% had achieved within 1.0 D. Data from case series of more than1800 eyes undergoing LASEK for myopia or astigmatism showed that a median of 75% of eyes were within 0.5 D and a median of 92% of eyes were within 1.0 D of their intended correction at 36 months follow-up. Data from eyes treated with LASIK for myopia or astigmatism showed that 77% (7309/9542) were within 0.5 D and 91% (8109/8885) were within 1.0 D of their intended correction at 312 months. One RCT found LASEK to be significantly more accurate than PRK for eyes with hyperopia. Question can you calculate NNT from case-series data? For example is the NNT for myopia correction within 0.5D with PRK calculated as 1/0.69= 1.45? If not, why not. RE NNH The guidance also has a section on safety that notes: In eyes treated for myopia, loss of two lines of best spectacle-corrected visual acuity was seen in a median of 0.5% (020.5%) of eyes treated with PRK, 0% (08.2%) of eyes treated with LASEK and 0.6% (03%) of eyes treated with LASIK. Patients with high myopia were more likely to lose two or more lines of best spectacle-corrected visual acuity than those with moderate to low myopia. Flap complications may occur during LASIK and LASEK, requiring conversion to PRK or postponement of ablation (with LASIK), and occasionally there may be loss of best spectacle corrected visual acuity. Epithelial in-growth was reported in LASIK in a median of 1.3% (0.04.4%) of eyes. Ectasia, a condition that can result from corneal thinning, is a serious complication related to refractive surgery that can lead to loss of vision. Data from the review estimated the risk of ectasia following LASIK as a median of 0.2% (range 0 to 0.87%; overall rate of 40/10,806 eyes). However, many of the affected eyes may have been selected inappropriately for LASIK treatment, and with appropriate patient selection the rate might have been lower. Rates of ectasia were not reported following PRK, and very little information was reported about LASEK, with no cases of ectasia described in one case series of 171 eyes. Microbial keratitis was only reported in LASIK studies and occurred in 00.16% of eyes. This incidence was similar to, or lower than, that reported for contact lens wearers. Other patient-reported problems included visual difficulty in low light conditions, corneal haze, light halos and problems with glare. Significant corneal haze was reported following all three procedures (in 031% of eyes treated with PRK, 025% with LASEK and 02% with LASIK). Glare and night vision difficulties were less common after LASIK. For more details, refer to the Sources of evidence. Is it fair to calculate NNH from data given here? e.g epithelial in growth with LASIK as NNH = 1/0.013=77?