A system for analyzing a fluid includes an in-line sensor configured to analyze a fluid flowing past the in-line sensor to determine at least one in-line value of a fluid parameter of the fluid across an event period, and a sample sensor configured to analyze a sample of fluid extracted from the flow of fluid during the event period, to determine sample value of the fluid parameter for the sample. At least one processor is provided, configured to determine a representative in-line value of the fluid parameter across the event period based at least in part on the at least one in-line value, and determine an overall representative value of the fluid parameter across the event period based on the representative in-line value, the sample value for the sample, and one or more of the in-line values corresponding to the time of extracting the sample, wherein determination of the overall representative value is based on an error correction value determined for the in-line sensor during the event period.