Differences between experimental units
The experimental units are usually not identical — they have characteristics that affect the response.
Speed of checkout operators
An experiment is to be conducted to estimate how much time is saved in supermarket checkouts by employing packers to fill groceries into bags for the customers. Sixteen checkout operators are available for use in the experiment and speeds are measured in seconds per dollar value of the purchased goods over a 3-hour period on Saturday. The checkout operators vary in their age and experience, as shown in the scatterplot below.
We initially consider the speeds of the checkout operators if none of the checkout operators use a packer. Even without packers, the speeds of the operators will vary and some of this variability is likely to be related to their age and experience.
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 speeds of the operators.
Click the y-x and the y-z rotation buttons and observe that speed is associated with both age and experience. (The operators who are older and have more experience tend to be faster.)
Click Repeat experiment to run the experiment with a similar group of checkout operators with the same ages and experiences.
Remember that we are not interested on the effect of age and experience on speed.
We want to use the data to assess the effect of using a packer on speed so the effect of age and experience only serves to complicate the experiment.