Based on the error figures obtained after laboratory tests over a wide set of operational rain gauges from the network of the Liguria region, the bias introduced by systematic mechanical errors of tipping bucket rain gauges in the estimation of return periods and other statistics of rainfall extremes is quantified. An equivalent sample size is defined as a simple index that can be easily employed by practitioner engineers to measure the influence of systematic mechanical errors on common hydrological practice and the derived hydraulic engineering design. A few consequences of the presented results are discussed, with reference to data set reconstruction issues and the risk of introducing artificial climate trends in the observed rain records.

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