412 Statistical Methods
The cell means are plotted against the assembly line factor using separate
lines for the shift factor. This is called an interaction plot; you’ll create one
later in this chapter.
Because there is a constant spacing of 12 between the two shifts, the lines
are parallel. The pattern of ups and downs for the p.m. shift is the same as
the pattern of ups and downs for the a.m. shift.
What if interaction is allowed? Suppose that the eight cell population
means are as follows:
A B C D
p.m. 295 235 175 200
a.m. 317 241 142 220
In this situation, the difference between the shifts varies from assembly
line to assembly line, as shown in Figure 10-14. This means that any infer-
ence on the shift effect must take into account the assembly line. You might
claim that the a.m. shift generally produces more microwaves, but this is
not true for assembly line C.Figure 10-13
Means plot
without an
interaction
effectFigure 10-14
Means plot
with an
interaction
effect