## Core Concept
The Russell periodontal index is a method used to assess and classify the severity of periodontal disease. It evaluates the periodontal status based on clinical parameters. This index is significant in periodontology for determining the extent of periodontal destruction.

## Why the Correct Answer is Right
The Russell periodontal index involves measuring the depth of periodontal pockets and assessing the attachment loss around teeth, which are critical indicators of periodontal disease progression. By evaluating these parameters, the index provides a comprehensive assessment of periodontal health status. The correct calculation involves specific criteria that categorize the periodontal status into different scores.

## Why Each Wrong Option is Incorrect
**Option A:** This option is incorrect because it does not accurately represent the method used to calculate the Russell periodontal index. Without specific details on the formula or method, it's clear that A does not align with established periodontal assessment tools.

**Option B:** This option is incorrect as it also does not accurately describe the calculation method for the Russell periodontal index. The actual method involves more specific clinical measurements.

**Option D:** This option is incorrect because it, too, fails to accurately represent the calculation of the Russell periodontal index. The correct method is more nuanced, involving detailed clinical assessments.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the Russell periodontal index is an important tool for epidemiologists and clinicians to classify periodontal disease. It helps in planning treatment and in assessing the outcome of periodontal therapy. Understanding the index and its application is crucial for managing periodontal disease effectively.

**Correct Answer:** C.