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Abstract
The influence of counting errors (including random and systematic error and mis-identifications) on estimations of the Trophic Diatom Index (TDI) are assessed by means of an inter-laboratory calibration study. Coefficients of variation (cv) between estimates of the TDI were generally < 10% and little improvement was observed when the number of valves counted was increased from 200 to 400. Higher levels of variation (> 30% in one instance) between counts were recorded for individual taxa. A slight decrease in cv was observed when a count of 400, rather than 200 valves, was used. However, this is unlikely to have any effect on the final value of the TDI.
