Weight gain of 18 calves over a 2-month period

Use this diagram to explain that the pool of experimental units available for an experiment usually includes units that differ from each other. Some of these differences may affect the response from the experiment that is conducted using them.

The diagram initially shows the ages and initial weights of a group of 18 calves that will be used in an experiment to assess whether a feed supplement improves the weight gain of calves over a 2-month period.

The third dimension shows weight gains for the calves that would happen if none are given the supplement. (Click Repeat experiment to repeat with another group of calves with the same ages and weights.)

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.)

Heavier calves are likely to gain most weight, as are older calves.