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Inter-observer and intra-observer differences in measuring body length: a test in the common lizard, Zootoca vivipara

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The snout-vent length (SVL), a conventional measure of overall body size in lizards and snakes, is used in a wide variety of ecological, evolutionary, and taxonomical studies. Trends in SVL variation are often analysed using data from several researchers (observers), but possible confounding effects due to inter-observer differences in measurement protocols have never been appropriately examined. This study reports inter-observer biases between eleven herpetologists who measured the same specimens of the Eurasian common lizards (21 adult specimens were examined by eight observers and additional 192 specimens by two observers). Intra-observer bias over time (1.5-15 months between measuring sessions) was also estimated. In the vast majority of comparisons, mean difference between the first author and another observer varied from −1.0 to +0.8 mm, or from −1.9 to +1.6% if expressed as a percent of the specimen's SVL value. Some non-regular effects of sex and study sample on the studied bias were revealed, and their possible reasons are discussed. We are advising the researchers who intensively collect SVL and other morphometric data to consider testing intra-observer and inter-observer biases and to establish etalon samples available for re-examinations.

Affiliations: 1: Institute of Integrated Sciences, Department of Biology, University of Koblenz-Landau, Universitätsstr. 1, D-56070 Koblenz, Germany;, Email:; 2: Zoological Research Museum, Moscow M. V. Lomonosov State University, Bolshaya Nikitskaya 6, 125009 Moscow, Russia; 3: Tomsk State University, prosp. Lenina 36, 634050 Tomsk, Russia; 4: Museum of Nature, Karazin Kharkiv National University, vul. Trinklera 8, 61022 Kharkiv, Ukraine; 5: Department of Evolutionary Biology, Institute for Biological Research, University of Belgrade, Bulevar Despota Stefana 142, 11060 Belgrade, Serbia; 6: Surgut State University, ul. Energetikov 14, 628400 Surgut, Russia; 7: CIBIO-UP, Centro de Investigação em Biodiversidade e Recursos Genéticos, Campus Agrário de Vairão, 4485-661 Vairão, Portugal; 8: Zoologisches Forschungsmuseum A. Koenig, Adenauerallee 160, D-53113 Bonn, Germany; 9: Department of Zoology, Charles University, Vinicná 7, CZ-12844 Praha 2, Czech Republic


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