Differences between experimental units
The experimental units are usually not identical — they have characteristics that affect the response.
Weight gain of calves
An experiment is to be conducted to assess whether a feed supplement improves the weight gain of calves over a 2-month period. Eighteen calves are available for use in the experiment. These calves vary in their age and weight at the start of the experiment, as shown in the scatterplot below.
We initially consider the weight gains of the calves if none of them are given the feed supplement. Even without being given a supplement, the weight gains of the calves will vary and some of this variability is likely to be related to the initial ages and weights.
The diagram is 3-dimensional, so move the mouse to the centre (marked by either a pink circle or the pointer changing to a hand) and drag towards the top left to rotate. (Or click the y-x-z rotation button.) The third dimension shows weight gains for the calves.
Click the y-x and the y-z rotation buttons and observe that weight gain is associated with both age and initial weight. (The calves that are older and heavier at the start of the experiment tend to gain more weight.)
Click Repeat experiment to run the experiment with a different group of 18 calves that have the same initial ages and weights.
Remember that we are not interested on the effect of age and initial weight on weight gain.
We want to use these calves to assess the effect of a feed supplement on weight gain so the effect of age and initial weight only serves to complicate the experiment.