Speed of checkout operators

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 experiences of a group of 18 checkout operators who will be used in an experiment to assess whether employing packers to fill groceries into bags for the customers improves the speed of the checkout operators.

The third dimension shows the speeds (seconds per dollar) for the checkout operators that would be recorded if none are given packers. (Click Repeat experiment to repeat the experiment with the same checkout operators.)

Click the y-x and the y-z rotation buttons and observe that speed is associated with both age and experience. (The older and more experienced operators take less time per dollar to process customers.)

Younger and less experienced operators tend to be slower.