The influence of process analyzer location and performance on plant-wide process monitoring was evaluated using five uncertainty contributions to the estimation error: measurement error/uncertainty, analysis frequency, sample size/grab error, analyzer memory effect/response correlation, and delay time. The choice of both the location and performance characteristics of different process analyzers can be evaluated using a measurability factor M, ranging from zero to one, where one indicates perfect monitoring capabilities. Due to the unifying nature of the measurability factor, this factor can be used to make a rational decision between very different process analyzers. This allows for finding optimal process analyzer configurations for existing processes or for processes in the design phase. A tubular reactor simulation model was used for the partial oxidation of benzene to maleic anhydride to demonstrate the use of the measurability factor.