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Sodium depletion and hemoconcentration: Overlooked complications in patients with anorexia nervosa?
Nutrition 21 (2005) 986 www.elsevier.com/locate/nut
Letter to editor
Sodium depletion and hemoconcentration: Overlooked complications in patients with anorexia nervosa? Yusuf Celik, Ph.D.a, and Fatma Celik, Ph.D.b a
Biostatistics Department, Faculty of Medicine Dicle University Diyarbakir, Turkey Nutrition and Dietetics Division Hospital of Dicle University Diyarbakir, Turkey
b
We have read the article entitled “Sodium depletion and Hemoconcentration: Overlooked Complications in Patients With Anorexia Nervosa?” in Nutrition (vol. 21, no. 4, April 2005). We would like to clarify the following points. In Table 2, the mean ⫾ standard deviation of the ratio of serum urea nitrogen to creatinine was presented as 33.91 ⫾ 18.68. According to this result, mean ⫾ two standard deviations will be equal to 33.91⫾ 37.36. The value of two standard deviations of the ratio of serum urea nitrogen to creatinine (37.36) is greater then its mean value (33.91). These results showed that the variables were not normally distributed and thus were not symmetrical. We think the use of a median value is more appropriate. Means and standard deviation are parameters of normal distribution and should be used only to describe continuous variables. The normal distribution is used when the sample is larger than 30 [1,2]. In the same way, Student’s t distribution is symmetrical. Therefore, Wilcoxon’s paired sample test is more appropriate than the paired sample t test that was used in the study. The correlation between an increase in body weight and
change in hemoglobin during fluid supplementation was displayed in Fig. 4. However, the authors did not mention anything about the correlation method in the STATISTICAL ANALYSIS. It is known that Pearson’s correlation coefficient is one of the parametric tests; therefore, Spearman’s correlation coefficient is more appropriate in this study [3]. In Table 3 the probability was presented as P ⫽ 0.0000, which is not appropriate to the logic of probability. Perhaps it should have been P ⬍ 0.0001. It needs to reorganize the outcomes of statistical computer packages according to statistical principles.
References [1] Banerjee A. Medical statistics made clear: an introduction to basic concepts. London: Royal Society of Medicine Press; 2003. [2] Toksoz P, Celik F, Ceylan A, Gokce. The fast food consumption status of high school students. Dicle Med J 2000;27:141–9. [3] Celik Y. Biostatistics, principles of research. Diyarbakir: Dicle University Press; 1999.
Reply
Lorenza Caregaro Negrin, Ph.D. Department of Clinical and Experimental Medicine, University of Padua, Padua, Italy
Reply: We agree with Drs. Yusuf and Fatma Celik that nonparametric tests are more appropriate in analyzing data of a small group of subjects, as in our study. The analyses were repeated using Wilcoxon’s paired sample test, and the results reported in Table 2 did not substantially change. The P values at discharge compared with those at admission were:
0899-9007/05/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.nut.2005.04.004
0.001 for hemoglobin, 0.003 for hematocrit, 0.01 for serum creatinine, and 0.03 for serum sodium. Other values were unchanged. The relation between increase in body weight and change in hemoglobin during fluid supplementation (Fig. 4) was analyzed with Spearman’s correlation coefficient, and the result did not change (r ⫽ 0.64, P ⬍ 0.014).
Letter to editor
Sodium depletion and hemoconcentration: Overlooked complications in patients with anorexia nervosa? Yusuf Celik, Ph.D.a, and Fatma Celik, Ph.D.b a
Biostatistics Department, Faculty of Medicine Dicle University Diyarbakir, Turkey Nutrition and Dietetics Division Hospital of Dicle University Diyarbakir, Turkey
b
We have read the article entitled “Sodium depletion and Hemoconcentration: Overlooked Complications in Patients With Anorexia Nervosa?” in Nutrition (vol. 21, no. 4, April 2005). We would like to clarify the following points. In Table 2, the mean ⫾ standard deviation of the ratio of serum urea nitrogen to creatinine was presented as 33.91 ⫾ 18.68. According to this result, mean ⫾ two standard deviations will be equal to 33.91⫾ 37.36. The value of two standard deviations of the ratio of serum urea nitrogen to creatinine (37.36) is greater then its mean value (33.91). These results showed that the variables were not normally distributed and thus were not symmetrical. We think the use of a median value is more appropriate. Means and standard deviation are parameters of normal distribution and should be used only to describe continuous variables. The normal distribution is used when the sample is larger than 30 [1,2]. In the same way, Student’s t distribution is symmetrical. Therefore, Wilcoxon’s paired sample test is more appropriate than the paired sample t test that was used in the study. The correlation between an increase in body weight and
change in hemoglobin during fluid supplementation was displayed in Fig. 4. However, the authors did not mention anything about the correlation method in the STATISTICAL ANALYSIS. It is known that Pearson’s correlation coefficient is one of the parametric tests; therefore, Spearman’s correlation coefficient is more appropriate in this study [3]. In Table 3 the probability was presented as P ⫽ 0.0000, which is not appropriate to the logic of probability. Perhaps it should have been P ⬍ 0.0001. It needs to reorganize the outcomes of statistical computer packages according to statistical principles.
References [1] Banerjee A. Medical statistics made clear: an introduction to basic concepts. London: Royal Society of Medicine Press; 2003. [2] Toksoz P, Celik F, Ceylan A, Gokce. The fast food consumption status of high school students. Dicle Med J 2000;27:141–9. [3] Celik Y. Biostatistics, principles of research. Diyarbakir: Dicle University Press; 1999.
Reply
Lorenza Caregaro Negrin, Ph.D. Department of Clinical and Experimental Medicine, University of Padua, Padua, Italy
Reply: We agree with Drs. Yusuf and Fatma Celik that nonparametric tests are more appropriate in analyzing data of a small group of subjects, as in our study. The analyses were repeated using Wilcoxon’s paired sample test, and the results reported in Table 2 did not substantially change. The P values at discharge compared with those at admission were:
0899-9007/05/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.nut.2005.04.004
0.001 for hemoglobin, 0.003 for hematocrit, 0.01 for serum creatinine, and 0.03 for serum sodium. Other values were unchanged. The relation between increase in body weight and change in hemoglobin during fluid supplementation (Fig. 4) was analyzed with Spearman’s correlation coefficient, and the result did not change (r ⫽ 0.64, P ⬍ 0.014).