Table 4.

Summary of the risk of bias for retrospective studies according to Newcastle–Ottawa Quality Assessment Scale criteria

Study	Representativeness of the exposed cohort	Selection of the nonexposed cohort from same source as exposed cohort	Ascertainment of exposure	Outcome of interest was not present at start of study	Comparability of Cohorts	Assessment of outcome	Follow-up long enough for outcome to occur	Adequacy of follow-up	Qualityscore
Farouk et al., 2020	Somewhat representative of the average malocclusion in the community*	Yes*	Secure report*	No	Comparison of tooth length measurements to detect root resorption*	Independent blind assessment*	Yes*	Complete follow-up*	Low riskof bias
Shipley et al., 2018	Somewhat representative of the average malocclusion in the community*	Yes*	Secure record*	No	Intergroup comparison of treatment duration and incidence of refinements*	Record linkage*	Yes*	Subjects lost to follow-up unlikely to introduce bias*	Low riskof bias
Shipley et al., 2020	Somewhat representative of the average malocclusion in the community*	Yes*	Secure record*	No	Intergroup comparison of aligner exchange interval, Intergroup and Intragroup comparison of bone density change*	Record linkage*	Yes*	Complete follow-up*	Low riskof bias

*Good Quality: 3 or 4 stars in Selection domain AND 1 or 2 stars in Comparability domain AND 2 or 3 stars in Outcome domain