Goats data story

Last edited on 27th April, 2004 by gs/ gs . - Host home page:http://www.statlab.uni-heidelberg.de 
Experiments were carried out on six commercial goat farms to determine whether the standard worm drenching program was adequate. Forty goats were used in each experiment. Twenty of these, chosen completely at random, were drenched according to the standard program, while the remaining twenty were drenched more frequently. The goats were individually tagged, and weighed at the start and end of the year-long study. For the first farm in the study the resulting liveweight gains are given along with the initial liveweights. In each experiment the main interest was in the comparison of the liveweight gains between the two treatments. This comparison could be made using an ANOVA as detailed in Chapter 6. However, a commonly observed biological phenomenon allows us to increase the precision of the analysis. This is the phenomenon of "regression to the norm", which in our instance means that the lighter animals gain more weight than the heavier animals. Since this can be assumed to occur within both treatment groups, it is appropriate to use ANCOVA to fit two parallel lines to the data, as shown in the figure. The corresponding ANCOVA comparison of adjusted mean liveweight gains will in general be more sensitive than the ANOVA comparison of unadjusted mean liveweight gains.
Weight gain versus initial weight for standard (blue) and intensive(red) drenching program

This example is based on a study conducted by Mr Alan Pearson, Veterinarian, Animal Health Laboratory, Lincoln, New Zealand.
liveweight gains, in kg
initial liveweights, in kg
treatment: standard or intensive drench program

Data file: goats data

Source: D.J. Saville, G. R. Wood: Statistical Methods: The Geometric Approach. Ch. 17. New York:Springer 1991.