Sample Size Analysis Sample size analysis estimates (hit and miss rates) were based on the results of observational studies. In a recent single center study of our group on the accuracy of ventricular catheter placement, using anatomic landmarks versus neuronavigation or CT-laser guidance, ventricles were hit in 69.2% of catheter insertions (31% miss rate) using anatomical landmarks (non-assisted), as compared to 82.3% with the use of guidance (neuronavigation or CT-guidance, p=0.043) (submitted data). The placements that were non-assisted were significantly more likely to be placed extraventricularly than those using guidance (OR 3.73, 95%, Confidence Interval (CI) 1.24, 11.19, p=0.019). Given data from other retrospective studies on ventricular catheters (in the context of ventricular-peritoneal shunts), similar rates between 25-45%4-7 of malpositioning are described.