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Serum citrulline levels in infants with short bowel syndrome
Edward Vincent Faustino, MD Michael Apkon, MD, PhD, MBA Section of Critical Care and Applied Physiology Department of Pediatrics Yale University School of Medicine New Haven, CT
Yusuf Celik, Prof Dr, PhD Biostatistics Department Faculty of Medicine Dicle University Diyarbakir, Turkey
10.1016/j.jpeds.2006.01.015
Fatma Celik, Asst Prof Dr, PhD Nutrition and Dietetics Division Hospital of Dicle University Diyarbakir, Turkey
Serum citrulline levels in infants with short bowel syndrome To the Editor: In the study by Rhodes et al,1 data were expressed as mean ⫾ SEM values. Means and standard errors were computed for each amino acid and total parental nutrition group. The groups were compared by means of two-sided t tests. Linear regressions were performed with serum citrulline as the response and percentage of enteral calories or bowel length as a predictor. In Table II, the mean ⫾ SEM value of ALA for Control (n ⫽ 21; 239.1; 70.5), for Off (n ⫽ 6; 378.6; 77.3), and so forth, were presented. The standard error of the mean is always smaller than standard deviation, and so its use makes measurements look more precise than they are. Most authors, however, fail to summarize their data with the standard deviation; they use the standard error of mean. Unlike the standard deviation, the standard error of the mean does not summarize the variability in the observations. In addition to this, mean and standard deviation values are the parameters of normal distribution, and the normal distribution is used when the sample size is larger than 30. In fact, the mean and the standard deviation should be used only to describe approximately normal distributions. By definition, approximately 67% of the values of a normal distribution are within ⫾1 SD of the mean, and approximately 95% are within ⫾2 SD.2,3 In this study, standard error of the mean was calculated instead of standard deviation. Because of the value of standard deviation for some variables will be greater than their mean value. We can compute the standard deviation by using the simple formula SD ⫽ SEM兹samplesize The value of standard deviation of ALA will be 323.07 by using the formula. This could be showed as X⫾SD ⫽ 239.1⫾323.07. Thus, we can indicate that the standard deviation (323.07) is greater then mean value (239.1). Finally, data are not normally distributed. Non-normal or skewed distributions, however, are not appropriately described with the mean and the standard deviation and analyzed by linear regression method (in the study). One of the stronger assumptions of the linear regression method is that the variables are normally distributed. Statistical tests may not give accurate results if their assumptions are violated.2,4 848
10.1016/j.jpeds.2005.06.015
REFERENCES 1. Rhoads JM, Plunkett E, Galanko J, Lichtman S, Taylor L, Maynor A, et al. Serum citrulline levels correlate with enteral tolerance and bowel length in infants with short bowel syndrome. J Pediatr 2005;146:542-7. 2. Lang T. Common statistical errors even you can find, I: errors in descriptive statistics and in interpreting probability values. AMWA J 2003;18:67-71. 3. Toksoz P, Celik F, Ceylan A, Gokce S. The fast food consumption status of high school students. Dicle Med J 2000;27:141-9. 4. Celik Y. Biostatistics, principles of research. Dicle University Press, Dicle, Turkey, 1999.
Reply To the Editor: We prefer presenting our data as mean ⫹ SEM, because the 95% confidence interval can be seen as mean ⫹ 2⫻ SEM.1 (This approach is valid only if the data are normally distributed.) We would like to thank Drs. Celik for bringing to our attention the SEM value for serum alanine in our subjects. We regret that an error was made in data expression, and the mean ⫾ SEM values for serum alanine (ALA) in the control subject (Table II of our study)1 should have been 239.1 ⫾ 15.4 mol/L. We also reanalyzed all of the amino acid data in Table II using a nonparametric test (Mann-Whitney) that compares medians. We found that 18 of the entries in Table II that are not different from control values when we use a t-test are statistically significant when we use the Mann-Whitney test. Conversely, only 1 entry in Table II that is statistically significant with a t-test (ie, comparison of control vs on– off for LYS) is not statistically significant when we use the MannWhitney test. Therefore, the results show even more statistically significant differences when a nonparametric test is used to compare the median values of amino acids. When we tested to see whether citrulline (CIT) values failed the Shapiro-Wilk test for normality, we found that CIT levels do not show “nonnormality” for any of the 4 groups in Figure 1 or the plot of enteral calories in Figure 2. In Figure 2 the points are spread evenly around a line, indicating that normality is not violated. In the graph with bowel length (Figure 3), there may be a reason to question the normality of CIT values in patients with shorter bowel length, but results of the linear correlation analysis indicate that serum CIT levels are significantly correlated with bowel length in our study subjects (R ⫽ 0.47; P ⱕ .03). It should be kept in mind that serum CIT The Journal of Pediatrics • June 2006
Yusuf Celik, Prof Dr, PhD Biostatistics Department Faculty of Medicine Dicle University Diyarbakir, Turkey
10.1016/j.jpeds.2006.01.015
Fatma Celik, Asst Prof Dr, PhD Nutrition and Dietetics Division Hospital of Dicle University Diyarbakir, Turkey
Serum citrulline levels in infants with short bowel syndrome To the Editor: In the study by Rhodes et al,1 data were expressed as mean ⫾ SEM values. Means and standard errors were computed for each amino acid and total parental nutrition group. The groups were compared by means of two-sided t tests. Linear regressions were performed with serum citrulline as the response and percentage of enteral calories or bowel length as a predictor. In Table II, the mean ⫾ SEM value of ALA for Control (n ⫽ 21; 239.1; 70.5), for Off (n ⫽ 6; 378.6; 77.3), and so forth, were presented. The standard error of the mean is always smaller than standard deviation, and so its use makes measurements look more precise than they are. Most authors, however, fail to summarize their data with the standard deviation; they use the standard error of mean. Unlike the standard deviation, the standard error of the mean does not summarize the variability in the observations. In addition to this, mean and standard deviation values are the parameters of normal distribution, and the normal distribution is used when the sample size is larger than 30. In fact, the mean and the standard deviation should be used only to describe approximately normal distributions. By definition, approximately 67% of the values of a normal distribution are within ⫾1 SD of the mean, and approximately 95% are within ⫾2 SD.2,3 In this study, standard error of the mean was calculated instead of standard deviation. Because of the value of standard deviation for some variables will be greater than their mean value. We can compute the standard deviation by using the simple formula SD ⫽ SEM兹samplesize The value of standard deviation of ALA will be 323.07 by using the formula. This could be showed as X⫾SD ⫽ 239.1⫾323.07. Thus, we can indicate that the standard deviation (323.07) is greater then mean value (239.1). Finally, data are not normally distributed. Non-normal or skewed distributions, however, are not appropriately described with the mean and the standard deviation and analyzed by linear regression method (in the study). One of the stronger assumptions of the linear regression method is that the variables are normally distributed. Statistical tests may not give accurate results if their assumptions are violated.2,4 848
10.1016/j.jpeds.2005.06.015
REFERENCES 1. Rhoads JM, Plunkett E, Galanko J, Lichtman S, Taylor L, Maynor A, et al. Serum citrulline levels correlate with enteral tolerance and bowel length in infants with short bowel syndrome. J Pediatr 2005;146:542-7. 2. Lang T. Common statistical errors even you can find, I: errors in descriptive statistics and in interpreting probability values. AMWA J 2003;18:67-71. 3. Toksoz P, Celik F, Ceylan A, Gokce S. The fast food consumption status of high school students. Dicle Med J 2000;27:141-9. 4. Celik Y. Biostatistics, principles of research. Dicle University Press, Dicle, Turkey, 1999.
Reply To the Editor: We prefer presenting our data as mean ⫹ SEM, because the 95% confidence interval can be seen as mean ⫹ 2⫻ SEM.1 (This approach is valid only if the data are normally distributed.) We would like to thank Drs. Celik for bringing to our attention the SEM value for serum alanine in our subjects. We regret that an error was made in data expression, and the mean ⫾ SEM values for serum alanine (ALA) in the control subject (Table II of our study)1 should have been 239.1 ⫾ 15.4 mol/L. We also reanalyzed all of the amino acid data in Table II using a nonparametric test (Mann-Whitney) that compares medians. We found that 18 of the entries in Table II that are not different from control values when we use a t-test are statistically significant when we use the Mann-Whitney test. Conversely, only 1 entry in Table II that is statistically significant with a t-test (ie, comparison of control vs on– off for LYS) is not statistically significant when we use the MannWhitney test. Therefore, the results show even more statistically significant differences when a nonparametric test is used to compare the median values of amino acids. When we tested to see whether citrulline (CIT) values failed the Shapiro-Wilk test for normality, we found that CIT levels do not show “nonnormality” for any of the 4 groups in Figure 1 or the plot of enteral calories in Figure 2. In Figure 2 the points are spread evenly around a line, indicating that normality is not violated. In the graph with bowel length (Figure 3), there may be a reason to question the normality of CIT values in patients with shorter bowel length, but results of the linear correlation analysis indicate that serum CIT levels are significantly correlated with bowel length in our study subjects (R ⫽ 0.47; P ⱕ .03). It should be kept in mind that serum CIT The Journal of Pediatrics • June 2006