In-line oil blending/certification, custody transfer, and performance monitoring are usually carried out with the help of analyzers that measure specific, and typically complex, properties or compositions, which must be reliable and verifiable for economic and regulatory reasons, as well as for safety and quality requirements. In these operations, in-line measuring devices can generate process data at very high frequencies in typical continuous industrial plants. For this reason, it is proposed here that variance spectra generated by in-line monitoring devices be used for validation of process measurements and implementation of quality control procedures, while laboratory practices/standards are used to support and evaluate the reliability of the proposed monitoring scheme on a regular basis. The illustrative application studies reveal the great influence that uncertainties and disturbances can exert upon decision-making in continuous chemical processes and how the proposed monitoring schemes can allow for improvement of process operation and identification of process failures.