Independent variable/experimental conditions
BLIND
Participants were randomized to one out of two conditions, differing with respect to the instruction they received prior to tooth brushing. These were either “brush your teeth as thoroughly as you can so that they are completely clean” (arm 1; best of one’s abilities (BP)) or “brush your teeth as usual” “(arm 2; as usual (AU)) (instructions are translated from German; for original German instructions, see For randomization, A2 drew a lot with the respective instruction out of an opaque box. A2 was kept blind regarding the results of the dental examinations, as were the dentists regarding the experimental condition of the participant. To stratify with respect to participants’ sex and the examining dentist, lots were distributed to four boxes (one box for each dentist and each sex).