Effect of a feed supplement on the weight gain of 18 calves
The first 9 calves to be caught were separated from the rest and given a feed supplement.
Use this diagram to explain how poor allocation of treatments to experimental units can result in the treatment effect being over- or under-estimated. Click Allocate treatments then Conduct experiment to see typical experimental results.
Since the animals getting the supplement were generally older and heavier, they would have had more weight gain even without getting the supplement (as explained in the diagram on the previous page). As a result, they have average weight gain that is 10 higher after two months than those getting the control diet even though the effect of the supplement is only 5.
The experiment has been badly designed.
Repeat a few times. To help explain, change the true effect of the supplement to zero with the pop-up menu and repeat, noting that the experiment estimates that the supplement does have an effect.