Meanwhile, across town, the competition was having trouble with its own production of the
same type of washer. The problem was that not enough washers had thicknesses that were within
specifications. As a first step in finding out why, production engineers took a sample
of 100 washers at random from the production line and measured their thicknesses. Their first
task was to decide whether the process was stable. As the
washers were manufactured by three different machines, they plotted the washer thicknesses
versus the order in which they were taken for each machine separately. The results are shown
in Figure 
. These plots give no evidence that production from any of the
machines is unstable. However, the plotted patterns do not all look the same either in their
location or spread.