Statistical methods
’s mouth, bleeding, Tooth, tooth
REGRESSION, BLEEDING, PLAQUE
Data analysis was carried out using JMP®, Version 14 Pro (SAS Institute, Inc., Cary, NC, 1989-2019) software. The data (PI, GI, and BoP) were tested for normality and an intention-to-treat analysis was applied. The three primary outcomes (plaque scores, gingival index, and bleeding on probing) were collected per tooth, at each time point. The data were structured such that the unit of analysis was the specific tooth ‘nested’ within the participant’s mouth, repeated over time (1, 6, and 12 months). This approach allowed the outcomes per individual tooth to be monitored in the mouth, permitting a more detailed analysis rather than averaging measurements per person which would mask upfront the majority of the variance. The interactions were tested using parameter estimates. The details of the models applied are described below and included coefficients per each comparison. Therefore, a mixed-effects (fixed and random) multiple linear regression approach was chosen. Using the random coefficients, this approach allowed for an elaborate error framework in accordance with the nested structure of the data without violating the independency assumption, using the Restricted Maximum likelihood (REML) approach instead of the classical repeated measures structure, the model was capable of dealing with any missing observations (when individual teeth were not present e.g. second molars) while balancing their weight with respect to the variance. In these models, time point (1-, 6-, and 12-month time points) and tooth were regarded as within participant effects, while toothbrush was the only between participant effect. Applying the full factorial mixed-effects multiple linear regression strategy for each independent variable (at the tooth level). The model estimated included the following fixed effects: Toothbrush type, Time point, Tooth, Toothbrush × Time point, Toothbrush × Tooth, Time point × Tooth, Toothbrush × Time point × Tooth. In addition, the models included the following random effects: Subject No., Subject No. × Time point, and Subject No × Tooth; each nested within toothbrush type. This allows the estimation of each individual tooth as well as all of them together at the patient level. The data were analysed on an intention-to-treat basis, with all participants included, according to their original allocation, regardless of the outcome of treatment.The reproducibility of measurements for the two investigators was assessed using the interclass correlation coefficient. For all analyses, a 