What is the II calculation?

• A. II = CV_G / CV_I
• B. II = CVA / CVG
• C. II = CV_I / CV_G
• D. II ≈ CV_I / CV_G

Answer: (D)

The concept being tested is the index of individuality, which is defined as the ratio of the within-subject variation to the between-subject variation. It tells you whether a given analyte is better interpreted against an individual's own baseline or against the population reference range. The two key terms are CV_I, the within-subject (intra-individual) coefficient of variation, and CV_G, the between-subject (inter-individual) coefficient of variation. In practice, we estimate this index from data as II ≈ CV_I / CV_G, since these CVs are themselves estimated from samples. A small II means an individual’s results are tightly clustered relative to how much the population varies, implying the subject’s own baseline is more informative; a large II means the population varies a lot and population reference values are more useful. The other proposed formulas mix up which variances are involved (for example, involving analytical variation) and thus don’t reflect the index of individuality. The approximate form is used because we’re dealing with estimated CVs from real data, not exact numbers.