Reliability of categorical versus continuous scoring of welfare indicators: lameness in cows as a case study

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Many animal welfare traits vary on a continuous scale but are commonly scored using an ordinal scale with few categories. The rationale behind this practice is rarely stated but appears largely based on the debatable conviction that it increases data reliability. Using 54 observers of varying levels of expertise, inter-observer reliability (IOR) and user-satisfaction were compared between a 3-point ordinal scale (OS) and a continuous modified visual analogue scale with multiple anchors (VAS) for scoring lameness in dairy cattle from video. IOR was significantly better for the VAS than for the OS. IOR increased with self-reported level of expertise for the VAS, whereas for the OS it was highest for observers with a moderate level of expertise. The mean continuous scores and the mean categorical scores were highly correlated. Three times as many observers stated a preference for the VAS (n = 27) compared to the OS (n = 9) in investigating differences in lameness between herds. Contrary to common perception, these results illustrate that it is possible for a continuous cattle lameness score to be more reliable and to have greater user acceptability than a simple categorical scale. As continuous scales are also potentially more sensitive, and produce data more amenable to algebraic processing and more powerful parametric analyses, the scepticism against their application for assessing animal welfare traits should be reconsidered.


Document Type: Research Article

Publication date: November 1, 2009

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