With a statistically designed experiment, it is demonstrated that gravimetric determinations of the oxygen-to-metal ratio of (U,Pu)O2 depend strongly on the day of analysis, slightly on the particular sintering run, and not detectably on position in either the sintering furnace or the analysis furnace. The average oxygen-to-metal ratio of a single sintering run can be determined to a precision of ±0.005 at the 95% confidence level.