Last edited on 27th April, 2004 by gs/ gs
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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.
Variables:
