Effect of a feed supplement on the weight gain of 18 calves
This diagram shows that random allocation of treatments to experimental units results in an unbiased estimate of the treatment effect — there is no tendency to over- or under-estimate.
Again click Allocate treatments then Conduct experiment to see typical experimental results. Observe that there is now no tendency for younger or older calves to get the supplement.
The estimated effect of the supplement is quite variable but there is no tendency to over- or under-estimate it.