Authors’ response

We understand Dr Ghazal’s concern about the combined sample used in this 2-center study. However, the Michigan Growth Study (Ann Arbor) and the Bauru Dental School Growth Study (Bauru, São Paulo, Brazil) have published 2 atlases representing the cephalometric standards of their respective underlying populations. Considering that the combined sample of this study was selected from this set of subjects and that their evaluated characteristics had similarities with other population studies, we have no reason to believe that the Michigan and Bauru subjects enrolled in this study were not representative of the population characteristics. Furthermore, the sample size was calculated to achieve a representative number of subjects in each group, and this procedure was based on statistical parameters of other population studies, which have also used combined samples, including subjects from the University of Michigan Elementary and Secondary Growth Study (Ann Arbor), the Broadbent-Bolton Collection at the Bolton-Brush Growth Study Center (Cleveland, Ohio), and the Denver Growth Study (Colo). Thus, our sample was as representative as those of other studies regarding population parameters.

Furthermore, even if a sample is selected from a single population and subsequently divided into groups, there is an actual likelihood that these groups will have statistical differences and heterogeneous variances, especially if the sample division into groups was not proportional. Because of the smaller number of available orthodontic records, the Bauru subjects represented about a third of the total study sample. In spite of this, the comparison of the cephalometric standards between the Michigan and Bauru subjects showed a high statistical similarity regarding the means and variances ( Table I ). Only 4 of the 21 evaluated variables had significant differences between the Michigan and Bauru subjects, and only 2 of the 21 variables showed heterogeneous variances ( Table I ). Thus, these populations do not seem to be as different as suspected, and this should not be highlighted as a significant limitation of this study. In fact, it can be speculated that a similar participation of each growth study center in the sample composition would produce an even greater variance homogeneity. Probably, the similar age, sex distribution, and ethnicity (most subjects had white ancestry) may have significantly contributed to this homogeneity ( Table II ). Perhaps this should have been mentioned in the article.

Table I
Cephalometric comparison between the Michigan and Bauru subjects with t tests and the Brown-Forsythe test of homogeneity of variances
Variable Michigan subjects, age 8.68 ± 0.75 y Bauru subjects, age 8.71 ± 0.97 y t test (P) Brown-Forsythe test (P)
Mean SD Mean SD
Maxillary skeletal components
SNA (°) 80.09 2.72 80.85 2.57 0.092 0.459
A-Nperp (mm) −0.14 2.24 0.30 2.01 0.211 0.430
CoA (mm) 79.56 4.26 80.70 3.89 0.102 0.791
Mandibular skeletal components
SNB (°) 75.94 2.51 76.66 3.12 0.120 0.080
SND (°) 72.82 2.38 73.68 3.0 0.054 0.081
P-Nperp (mm) −6.14 4.54 −5.02 3.51 0.116 0.013
CoGn (mm) 98.74 5.62 99.98 4.73 0.166 0.670
Co-Go (mm) 45.75 3.26 47.84 2.95 0.001 0.338
Go-Gn (mm) 64.57 4.28 65.22 2.98 0.318 0.051
Maxillomandibular relationship
ANB (°) 4.15 2.48 4.19 1.89 0.912 0.126
Growth pattern
SN.GoGn (°) 34.37 4.06 33.36 4.60 0.158 0.764
BaN.PtGn (°) −2.20 3.38 −1.79 4.04 0.502 0.497
SN.PP (°) 6.93 2.78 8.31 2.42 0.002 0.634
FH.MP (°) 26.57 4.10 26.03 3.66 0.418 0.418
LAFH/PFH (%) 93.67 6.94 91.03 6.47 0.021 0.989
Maxillary dentoalveolar components
Mx1-NA (mm) 3.89 1.92 2.91 1.65 0.001 0.175
Mx1.NA (°) 25.80 6.08 24.22 6.01 0.121 0.808
Mx1.SN (°) 105.90 5.53 104.08 5.92 0.055 0.778
Mandibular dentoalveolar components
Md1-NB (mm) 4.55 2.13 4.57 1.88 0.951 0.120
Md1.NB (°) 26.19 6.64 26.19 4.97 0.995 0.041
IMPA (°) 95.88 6.34 96.16 4.63 0.767 0.253
Only gold members can continue reading. Log In or Register to continue

Apr 4, 2017 | Posted by in Orthodontics | Comments Off on Authors’ response
Premium Wordpress Themes by UFO Themes