Experience with a certain type of plastic indicates that a relation exists between the hardness (measured in Brinell units) of items molded from the plastic (y) and the elapsed time since termination of the molding process (x). In a study to examine this relationship, 16 batches of the plastic were made, and from each batch, 1 test item was molded (Kutner et al. 2004; data slightly modified for ease of hand calculation without affecting results of the analysis). Each test item was randomly assigned to one of four predetermined time levels, and the hardness was measured after the assigned time had elapsed. The results are shown as follows: