Which statement about precision is true?

• A. The deterministic operation of the instrument
• B. Temperature is the only factor affecting precision
• C. The statistical nature of the detection process
• D. Precision is the same for all instruments

Answer: (C)

Precision is about how consistently you can reproduce measurements under the same conditions. In XRF, each measurement is a count of photons detected from the sample. Those photon counts fluctuate randomly because of the inherent randomness of the detection process, described by Poisson statistics. The uncertainty in the count is the square root of the number of counts, so the relative precision is 1 divided by the square root of the counts. For example, 10,000 counts give about 1% relative precision, while 100 counts give about 10%.

This is why the statement focusing on the statistical nature of the detection process is true: the random nature of photon counting sets the fundamental limit on precision. Other factors can influence measurements (like drift or averaging across measurements), but the primary source of repeatability limits in XRF is the counting statistics.