1. J. V. Bradley, 1968 . Distribution-free Statistical Tests . Prentice-Hall, Englewood Cliffs N. J.p.57 .
2. J. V. Bradley, 1968 . Distribution-free Statistical Tests . Prentice-Hall, Englewood Cliffs, N.J.p.94 .
3. Evaluation of an unsupervised oral hygiene programme
4. The Gingival Index, the Plaque Index and the Retention Index Systems