Calculating trend-indices for amphibian monitoring. In the Netherlands, amphibian species have been monitored by volunteers since 1997. Data of all species are collected in fixed sites that are spread over the country. ravon coordinates the fieldwork, while Statistics Netherlands uses the collected data to calculate trends, yearly indices and corresponding standard errors. In order to perform the analysis, one has to cope with problems like missing values, variability in counts and non-representative distribution of sites. Missing values are estimated 199using available data and the statistical package trim. Variation in survey techniques is minimised by using standardised fieldwork protocols. Count results are transformed into yearly values, using the annual maximum of a species abundance class for adults in individual waters. For some species the annual maximum abundance class for a cluster of waters is used, whereas for some rare species, the maximum number is used instead of maximum abundance class. When plots are not evenly presented over the distribution areas, the resulting trends may be biased towards oversampled areas. For national trend calculation, a correction procedure is built in. It uses weight factors per region, representing the species presence for that area relative to the national population. A weight factor is calculated using the total number of grid cells (one km2) with observations in a region and mean number of observations of the species per grid cell. In this article, the calculation of weight factors is described in detail. Trends and indices using weight factors are only slightly different from unweighted trends and indices. Differences are largest for Bufo calamita, which shows a faster decreasing population when count data are weighted per region. Weighted and unweighted trends and indices for the Dutch amphibians are presented in table 3.

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Tom van der Meij, Arco van Strien, Gerard Smit, & Edo Goverse. (2009). Trendberekening bij het meetnet amfibieën. RAVON, 10(4), 57–62.