How does a Z-score change?

• A. It remains constant
• B. It increases or decreases by 2 SD
• C. It increases or decreases by 1 SD
• D. It is reset each year

Answer: (C)

The Z-score is a statistical measurement that compares an individual's bone mineral density (BMD) to the average BMD of a population of the same age and sex. It indicates how many standard deviations a subject's BMD is from the mean value for that reference group.

When considering how a Z-score changes, it is important to note that it reflects the variability of BMD relative to peers. As a person ages, their BMD may decrease due to natural aging or health conditions, but the population mean also changes over time due to factors like improved nutrition or new data from larger studies. The Z-score is calculated based on the current BMD, the mean BMD of the reference population, and the standard deviation of that population.

When using statistical terminology, a change represented as an increase or decrease by 1 standard deviation indicates a significant shift in the population's BMD average or the individual’s BMD itself. Consequently, the Z-score adjusts to reflect these new values accurately, allowing for a correct interpretation of an individual's BMD status in relation to the reference group.

Thus, the nature of the Z-score is such that it is influenced by shifts in BMD measurements and population data over time, which is why it is expected to change by