Balanced study groups in a randomized trial - authors' response
© BioMed Central Ltd 2011
Published: 8 December 2011
Thank you for giving us the chance to reply to the comment raised by Dr Munis . As we wrote in the Materials and methods section of our article , we used a sealed envelope, simple randomization procedure in a 1:1 proportion of intervention to control. No stratification was performed, although there are some factors affecting the study group homogeneity.
The strict inclusion and exclusion criteria resulted in a narrow spectrum of patients in the study: 'healthier' patients were not entered, and 'sicker' patients usually underwent a less invasive procedure. Also, disease prevalence is not independent in such a homogenous group; for instance, patients undergoing abdominal aortic surgery (comprising 66% of included patients) were mostly men aged over 60 years and smokers. The association with peripheral artery disease is given by the procedure itself; hypertension, ischemic heart disease and pulmonary pathologies are common. When the procedure distribution is balanced, the same could be expected with associated variables. This can affect similar balancing observed in other optimization studies [3–5].
Chronic disease conditions
(N = 60)
(N = 60)
Pancreatitis in anamnesis
Pulmonary embolism in anamnesis
Chronic liver disease (usually mild to severe)
In conclusion, though our study was not intentionally stratified, the population homogeneity and dependence on disease prevalence could create some sort of 'pre-stratification'. With the inclusion of the rare conditions, however, the groups are not that balanced. We hope our explanation helps to elucidate and answer the problem raised by Dr Munis.
The study was supported by a research grant of Czech Ministry of Education, MSM0021620819.
- Request for clarification of randomization method[http://ccforum.com/content/14/3/R118/comments]
- Benes J, Chytra I, Altmann P, Hluchy M, Kasal E, Svitak R, Pradl R, Stepan M: Intraoperative fluid optimization using stroke volume variation in high risk surgical patients: results of prospective randomized study. Crit Care 2010, 14: R118. 10.1186/cc9070PubMed CentralView ArticlePubMedGoogle Scholar
- Van der Linden PJ, Dierick A, Wilmin S, Bellens B, De Hert SG: A randomized controlled trial comparing an intraoperative goal-directed strategy with routine clinical practice in patients undergoing peripheral arterial surgery. Eur J Anaesthesiol 2010, 27: 788-793. 10.1097/EJA.0b013e32833cb2ddView ArticlePubMedGoogle Scholar
- Boyd O, Grounds RM, Bennett ED: A randomized clinical trial of the effect of deliberate perioperative increase of oxygen delivery on mortality in high-risk surgical patients. JAMA 1993, 270: 2699-2707. 10.1001/jama.1993.03510220055034View ArticlePubMedGoogle Scholar
- Mayer J, Boldt J, Mengistu A, Rohm K, Suttner S: Goal-directed intraoperative therapy based on autocalibrated arterial pressure waveform analysis reduces hospital stay in high-risk surgical patients: a randomized, controlled trial. Crit Care 2010, 14: R18. 10.1186/cc8875PubMed CentralView ArticlePubMedGoogle Scholar