Methods and systems for determining a correction factor for use in monitoring network traffic, said network traffic comprising a plurality of flows of data units, each flow having associated therewith a flow identifier, said data units having associated therewith a path characterisation metric; the method comprising the steps of: selecting a subset of flow identifiers from the overall set of flow identifiers; determining, in respect of flows having associated therewith flow identifiers from said selected subset of flow identifiers, which flows if any are persistently irregular flows; determining a first measure of the total of the accumulated path characterisation metrics associated with data units of said selected subset of flows; determining a second measure indicative of the proportion of said total measure contributed by said persistently irregular flows; and determining a correction factor in dependence on the first and second measures.