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  1. Moderately preterm and late preterm infants:brief history

    I read with interest the recent article (1) wherein the authors use variable definitions for moderately preterm (MP) infants as 32-35 weeks of gestational age (GA) and 32-36 weeks GA. In the methods however , the authors categorize infants born at 32-35weeks gestation as MP infants and do not include 36 0/7-36 6/7 weeks of gestation due to study design. Upon review of the brief history of this terminology, it was found that until 2005, the National Center for vital statistics reported MP infants as infants 32-36wks GA(2,3). The National Institutes of Health (NIH) in 2005 recommended that births between 34 and 37 completed weeks GA be referred to as late preterm (LP) (4). The most recent definition of moderately preterm infants, however, is infants 32 0/7-33 6/7 weeks GA(5). In the study methods, including some of the LP infants (34 0/7-35 6/7 weeks GA) as MP infants causes confusion for the readers, especially when comparing outcomes from other studies. Interestingly, the National Center for vital statistics uses the definition 'very preterm' for infants <32 0/7 weeks GA, 'early preterm' for infants <34 0/7 weeks GA and LP for infants 34 0/7-36 6/7weeks GA(6). Also, in this paper, children born at term included 38-41 weeks GA, however the WHO definition of term infants includes births at 37 0/7-41 6/7 weeks GA. Excluding the children born at 37 0/7-37 6/7 weeks GA(which is a subset of early term infants) from the term cohort is also not ideal. Due to ongoing concerns regarding outcomes of these preterm infants, the use of consistent terminology is critical to designing further studies. I would like to make a humble plea and call for standardized nomenclature that is accepted worldwide.

    References

    1. Potijk MR, De Winter AF, Bos AF, Kerstjens JM, Reijneveld SA. Higher rates of behavioural and emotional problems at preschool age in children born moderately preterm. Arch Dis Child 2012;97:112-117. 2. Martin JA, Hamilton BE, Sutton PD, et al. Births: final data for 2003. Natl Vital Stat Rep. 2005;54:1-116 3. Davidoff MJ, Dias T, Damus K, et al. Changes in the gestational age distribution among U.S. singleton births: impact on rates of late preterm birth, 1992 to 2002. Semin Perinatol 2006;30:8-15 4. Raju TN. Epidemiology of late preterm (near-term) births. Clin Perinatol 2006; 33(4):751-63 5. Harijan P, Boyle EM. Health outcomes in infancy and childhood of moderate and late preterm infants. Semin Fet Neonat Med 2012;17(3):159-62. 6. Martin JA, Hamilton BE, Ventura SJ, et al. Births: final data for 2009. Natl Vital Stat Rep.2011;60:1-70

    Conflict of Interest:

    None declared

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  2. Re:Statistical vs. Clinical Significance

    We fully agree with Dr Miller, who draw attention to the fact that statistical significance is not always equal to clinical significance. We also agree that the effect size is the appropriate measure for clinical relevance. For the difference in mean total problems scores on the Child Behavior Checklist (CBCL) between moderately preterm and term-born children the effect size is 0.22 in our study, being a small (but not negligible) effect. However, clinicians in particular take care for those children that have elevated (clinical) CBCL scores, which have been presented in Table 4. In this Table, the effect sizes for total, externalizing, and internalizing problems are 0.34, 0.27, and 0.50, meaning small (0.34 and 0.27) to moderate effects (0.50). An effect size of 0.5 is often the value to be detected in clinical trials. However, we think that the effects are clinically relevant indeed because of the high prevalence of moderate preterm birth, which implies rather large effects on child public health. We thank Dr Miller for giving us the opportunity to provide this additional information on the relevance of our findings.

    Conflict of Interest:

    None declared

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  3. Statistical vs. Clinical Significance

    The interesting and well-conducted study of Potijk et al reminds us once again (though we probably don't need reminders) of the important difference between statistically versus clinically significant differences in research studies. The authors report that moderately preterm-born children had significantly worse scores on all subscales of the CBCL than did term born children; inspection of the P values in Table 2 shows that, statistically, this is quite correct. What is not discussed in the paper, however, is the clinical significance of these differences. A commonly used metric for evaluating the clinical significance of observed differences is Cohen's effect size coefficient 'd'. This 'd' is the ratio of the mean difference in scores between two groups to the standard deviation in scores of the groups. Most of the differences shown in Table 2 have a 'd' value of 0.2 or less, which by convention would be considered at the lower end of a small effect size. None of this takes away from the finding that there were differences between the groups, but it is important for readers (and authors) to consider the clinical significance of differences. The use of large study samples can make differences that are small and even trivial clinically, appear quite impressive statistically.

    Conflict of Interest:

    None declared

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