The null hypothesis is the proposition that implies no effect or no relationship between phenomena. It is a hypothesis that the researcher tries to disprove, reject, or nullify, whereas the alternative hypothesis is what the researcher really thinks is the cause of a phenomenon.
In the article “Evaluation of skeletal and dental asymmetries in Angle Class II subdivision malocclusions with cone-beam computed tomography,” the authors presented 2 null hypotheses. The first was that no significant difference exists between the Class II and Class I sides for the skeletal and dental measurements of Class II subdivision malocclusions. The second hypothesis was that there would be no significant difference in skeletal or dental measurements between the 2 sides when Class II subdivision malocclusions were separated into a noncrowded group with minimal or no dental crowding and a crowded group with moderate to severe dental crowding. Thus, the purpose of their study was to determine whether Angle Class II subdivision malocclusions have skeletal or dental asymmetries between the Class II and Class I sides. The null hypothesis is often the reverse of what the experimenter actually believes; it is put forward to allow the data to contradict it. However, in this particular case, rejection of 1 null hypothesis was already evident before the study began, because the authors recognized in their introduction that “there is asymmetry between the right and left sides of the dentition…. The etiology of the asymmetry can be quite complex. It could be dental related, skeletal related, or a combination of both…. These studies showed that the differences between the 2 sides were primarily dentoalveolar.”
We believe that the great merit of their study was to evaluate skeletal structures with cone-beam computed tomography in a way that was not possible before with 2-dimensional images. The ability of this technique to show spatial relationships in 3 dimensions helps the orthodontist to diagnose the relative location of the anatomic parts of the craniofacial complex. The conclusion that “there were significant skeletal and dental differences between the Class I and Class II sides” included the skeletal component in the dentoalveolar asymmetries, which have already been widely studied and discussed. Perhaps that is the most appropriate null hypothesis to be used: the role of skeletal asymmetries in the development of the Class II subdivision malocclusion.