et al. 1 an omission was published on page 2 of early view; "Each operator performed both needle placement into the lumbar subarachnoid space techniques in close succession with an order previously defined." After having received comments from a reader, the authors consider that the following sentences should have been included for clarification in the Materials and Methods section:. . . A total of 67 students performed both needle placement techniques in close succession. Forty-six students performed the blind landmark-guided technique first and then an additional group of 21 students was recruited after completion of this first study based on statistician recommendations for measuring and minimizing the effect of the order of the procedures on success rate. These 21 students started the procedure performing the ultrasound-guided technique first.Additional statistical analysis (chi-square test) was, therefore, performed to compare success rates of these two groups of students (those who started with the blind landmark-guided procedure and those who started with the ultrasound-guided procedure) . . .The authors consider that the following sentences should have been included in the Results section: . . . The success rate of the ultrasound-guided procedure was 72% for students who started with the blind landmarkguided method (group of 46 students) and 86% for students who started with the ultrasound-guided method (group of 21 students) with no statistical difference in success rate for the ultrasound-guided procedure between the two groups (P-value: 0.18).The success rate of the blind landmark-guided procedure was 35% for students who started with the blind landmarkguided method and 77% for students who started with the ultrasound-guided technique. This difference in success rate is statistically significant (P-value = 0.001) . . . The authors consider that the following sentences should have been included in the Discussion section: . . . One of the limitations of the study was that the number of students having performed the blind technique first is higher (46) compared to the number of students having performed the ultrasound-guided technique first (21), and that the order of procedures has not been randomized. Therefore, in order to avoid bias related to the experience potentially acquired while doing the procedure the first time and to the order of the procedures, the cofactor order was introduced in multinomial regression and the model has been adjusted for possible bias created by this variable. Results showed that the variable order was not significant in the multinomial regression (see Table 2 in the Results section).Moreover, success rate of ultrasound-guided needle placements was not statistically different between the two groups of students (72% vs. 86%) having performed the two techniques in a different order and it was higher in the groups of students having performed the ultrasound-guided procedure first. This result suggests that realization of a unique blind landmarkguided needle placement into the...