Of Donkeys and Nomograms

If you are a vet, you will often need to know the weight of an animal, for example to prescribe the right amount of a drug. If your animal is a rabbit, this is not a problem. But if your animal is large, say a donkey, and if you are out in the field, then you will not be able to weigh the animal directly. However, if you have a tape measure then you can weigh the animal indirectly, if you have a tool for converting tape measurements into weights. In this article we describe how we constructed such a tool—a nomogram—for Kenyan donkeys. Nomograms have been used before to predict weight on the basis of simpler measurements, including for horses, mules, and donkeys (see, e.g., Eley and French, 1993; Carroll and Huntington, 1988; Pearson and Ouassat, 1996; Kay et al., 2004). What we present here is a ‘more statistical’ treatment, which we hope can serve as a template for other similar studies. We consider a richer set of possible models, an appropriate loss function for choosing between them, the constraints of practical usage, and a careful assessment of accuracy. Our analysis was performed in the statistical computing environment R (R Core Team, 2013). All of our code and data is available on-line: see the Resources section at the end of the article. ∗Corresponding author: email [email protected].