You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is The Russell periodontal index is calculated by: and explain in detail?
The Periodontal Index (PI) was developed by Russell A.L. in 1956, over a trial period of ten years.
The Russell's rule states that "When in doubt assign the lesser score.
The Periodontal Index score (PI score) per individual is obtained by adding all of the individual scores and dividing by the number of teeth present or examined. 
PI score per person = Sum of individual scores/Number of teeth present.
Ref: Soben peter, 5th edition 
pg no:326