An article on dentofacial development and early mandibular incisor crowding in the August 2016 issue was done in an excellent manner; especially, the statistical part of the study was so informative (Barros SE, Chiqueto K, Janson G. Impact of dentofacial development on early mandibular incisor crowding. Am J Orthod Dentofacial Orthop 2016;150:332-8). The article highlighted a few new concepts contradicting previous study findings regarding mixed dentition crowding and the outcome in the early permanent dentition. I have a query regarding the type of study to which it belongs. The first line of the abstract mentions that it was retrospective longitudinal study. The records (study models and lateral cephalograms) of the subjects were analyzed at T1 (early mixed dentition) when the mean age was 8.66 ± 0.83 years. Records of the same subjects were analyzed again at T2 (early permanent dentition) when the mean age was 13.25 ± 1.19 years and the observation period was 4.58 ± 1.10 years. If there is follow-up of the subjects for a mean period of 4.58 years, then it should be a longitudinal study. If all the records of T1 and T2 were analyzed at the same time, then it should be classified as a retrospective cohort study. I believe that retrospective studies cannot be longitudinal, and longitudinal studies cannot be retrospective.
Longitudinal vs retrospective studies
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