The absence of difference between two groups is an important point. Commonly, retrospective studies include biases and confounders. Biases are specific of the retrospective nature and cannot be modified, whereas confounders may be minimized and data adjusted. In the current study, two groups had similar characteristics as in a randomized study. This figure is totally atypical for a retrospective design. Commonly, data are showed as crude differences and after adjustment for confounders!