Potential biases in the classification, analysis and interpretations in cross-sectional study: commentaries – surrounding the article "resting heart rate: its correlations and potential for screening metabolic dysfunctions in adolescents" - Moraes, Augusto César Ferreira de; Cassenote, Alex Jones Flores; Moreno, Luis A.; Carvalho, Heráclito Barbosa (BMC Pediatrics, 2014)