Non-parametric tests and ridits.
D003777:Dental Prophylaxis; D005500:Follow-Up Studies; D005891:Gingivitis / Q000175:diagnosis / Q000517:prevention & control; D006801:Humans; D008433:Mathematics; D009911:Oral Hygiene Index; D010512:Periodontal Index; D012107:Research Design; D013223:Statistics as Topic