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. 1. This study aims to develop a new method for assigning rarity weights to species in evaluations of the relative rarity of arthropod assemblages in conservation/monitoring studies. 2. A flexible characteristic was included in the rarity weighting method by introducing the possibility of fitting the method to a rarity cut-off point defined as the threshold of occurrence below which species are considered as being rare. This allows calculation of a rarity metric (index of relative rarity IRR) with multiple rarity cut-off points. 3. The proposed weighting method was used and compared with three previously proposed methods in a theoretical analysis. IRR values were then calculated for spider assemblages of a National Nature Reserve in France. Two methods of rankings were proposed: a local ranking between sites of the Nature Reserve, and a regional ranking in comparison to a reference database. 4. The proposed weighting method consistently weighted species according to the chosen rarity cut-offs. Species weights were less biased toward common species and rare species weights were less dispersed than with previous methods. Assemblages were consistently ranked according to the rarity of spiders in each assemblage. The index showed different patterns of rarity in assemblages which could not be detected by previous rarity metrics. 5. This method provides an improved understanding of assemblage rarity patterns relative to previous methods and can be consistently applied to other arthropod taxa in other geographic area and/or spatial scales. [ABSTRACT FROM AUTHOR]