Antibiotic and weight gain of calves
This simulation shows that the effect of a treatment (giving an antibiotic to calves) is estimated more accurately in a randomised block experiment than in a completely randomised experiment (at least provided the blocks differ from each other).
The top half of the diagram conducts a completely randomised experiment and the bottom half conducts a randomised block experiment. The ticks on the calves represent those getting the antibiotic. Click Repeat several times for each of the experiments.
Observe that the estimate of the effect of the antibiotic (the difference between the mean weight gain for the two treatments) is much more accurate (less variable)for the randomised block experiment.
(The differences between the blocks (herds) and the underlying effect of the treatment can be varied with the sliders at the top of the diagram.)
The context is an experiment that will be conducted on 24 calves, 8 of which are in each of three herds. (In practice, herd sizes would be much larger but this small-scale example illustrates the experimental designs more clearly.) The aim of the experiment is to estimate the effect on the weight gain of the calves of injection with an antibiotic.