This paper concerns the validity of a widely used method for estimating the architecture-level mean time to failure (MTTF) due to soft errors. The method first calculates the failure rate for an architecture-level component as the product of its raw error rate and an architecture vulnerability factor (AVF). Next, the method calculates the system failure rate as the sum of the failure rates (SOFR) of all components, and the system MTTF as the reciprocal of this failure rate. Both steps make significant assumptions. We investigate the validity of the AVF+SOFR method across a large design space, using both mathematical and experimental techniques with real program traces from SPEC 2000 benchmarks and synthesized traces to simulate longer real-world workloads. We show that AVF+SOFR is valid for most of the realistic cases under current raw error rates. However, for some realistic combinations of large systems, long-running workloads with large phases, and/or large raw error rates, the MTTF calculated using AVF+SOFR shows significant-discrepancies from that using first principles. We also show that SoftArch, a previously proposed alternative method that does not make the AVF+SOFR assumptions, does not exhibit the above discrepancies.