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CHAPTER 4 SOFTWARE PROCESS AND PROJECT METRICS 103

FIGURE 4.9 4
Differences in successive E r values 2.5
Moving range UCL = 5.57 (not shown)
control chart 3.5 mR bar
3

2
1.5
1
0.5
0
1 3 5 7 9 11 13 15 17 19
Projects

FIGURE 4.10 6
E r , errors found/review hour 4 1 (std. deviation)
Individual 1
control chart 5

A
m
3
2

1

0
1 3 (2 lines not shown) 11 13 15 17 19
5
7
9
Projects
lines is less than 0.0, it need not be plotted unless the metric being evaluated
takes on values that are less than 0.0.
WebRef
Applying these steps to the data represented in Figure 4.8, we derive an individual
The Common Control
Chart Cookbook covers control chart as shown in Figure 4.10.
the topic at some length Zultner [ZUL99] reviews four criteria, called zone rules, that may be used to eval-
and can be found at uate whether the changes represented by the metrics indicate a process that is in
www.sytsma.com/
tqmtools/ control or out of control. If any of the following conditions is true, the metrics data
ctlchtprinciples.html indicate a process that is out of control:
1. A single metrics value lies outside the UNPL.
2. Two out of three successive metrics values lie more than two standard devia-
tions away from A .
m
3. Four out of five successive metrics values lie more than one standard devia-
tion away from A .
m
4. Eight consecutive metrics values lie on one side of A .
m