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8 - PROJECT QUALITY MANAGEMENT

• Histograms, are a special form of bar chart and are used to describe the central tendency, dispersion, and
shape of a statistical distribution. Unlike the control chart, the histogram does not consider the influence
of time on the variation that exists within a distribution.
• Control charts, are used to determine whether or not a process is stable or has predictable performance.
Upper and lower specification limits are based on requirements of the agreement. They reflect
the maximum and minimum values allowed. There may be penalties associated with exceeding the
specification limits. Upper and lower control limits are different from specification limits. The control
limits are determined using standard statistical calculations and principles to ultimately establish the
natural capability for a stable process. The project manager and appropriate stakeholders may use the
statistically calculated control limits to identify the points at which corrective action will be taken to
prevent unnatural performance. The corrective action typically seeks to maintain the natural stability of a
stable and capable process. For repetitive processes, the control limits are generally set at ±3 s around
a process mean that has been set at 0 s. A process is considered out of control when: (1) a data point
exceeds a control limit; (2) seven consecutive plot points are above the mean; or (3) seven consecutive
plot points are below the mean. Control charts can be used to monitor various types of output variables.
Although used most frequently to track repetitive activities required for producing manufactured lots,
control charts may also be used to monitor cost and schedule variances, volume, and frequency of scope
changes, or other management results to help determine if the project management processes are in
control.

• Scatter diagrams, plot ordered pairs (X, Y) and are sometimes called correlation charts because they seek
to explain a change in the dependent variable, Y, in relationship to a change observed in the corresponding
independent variable, X. The direction of correlation may be proportional (positive correlation), inverse
(negative correlation), or a pattern of correlation may not exist (zero correlation). If correlation can be
established, a regression line can be calculated and used to estimate how a change to the independent
variable will influence the value of the dependent variable.

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