If II is above what value is population based RI sufficient?

• A. II < 0.6
• B. II > 1.4
• C. II = 1.0
• D. II between 0.6 and 1.4

Answer: (B)

The idea being tested is how the index of individuality informs when a population-based reference interval is appropriate. This index compares how much values vary between different individuals (between-subject variation) to how much a given individual’s values vary over time (within-subject variation). When the index is high—exceeding the threshold here of 1.4—the between-subject differences dominate and an individual’s day-to-day variation is relatively small, so a single population-based reference interval is sufficient to interpret results across the population. In other words, most people fall clearly on their own side of the population range, and you don’t gain much by chasing an individual’s personal baseline.

If the index were lower, within-subject variation would be more impactful, making a single population reference interval less reliable and increasing the value of tracking an individual’s own baseline or using multiple measurements. The 0.6 to 1.4 range is a gray zone, but the key takeaway is that when II is above 1.4, population-based RI is considered sufficient.