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8.3 Quality Control
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Quality control involves monitoring specific project results to determine if they
comply with relevant quality standards, and identifying ways to eliminate causes of
unsatisfactory results. It should be performed throughout the project. Project results
include both product results, such as deliverables and project management results,
such as cost and schedule performance. Quality control is often performed by a Quality Control
Department or similarly titled organizational unit, but it does not have to be.
The project management team should have a working knowledge of statistical
quality control, especially sampling and probability, to help it evaluate quality
control outputs. Among other subjects, the team may find it useful to
know the differences between:
Prevention (keeping errors out of the process) and inspection
(keeping errors out of the hands of the customer). Attribute sampling (the result conforms or it does not) and variables
sampling (the result is rated on a continuous scale that measures the degree of conformity). Special causes (unusual events) and random causes
(normal process variation). Tolerances (the result is acceptable if it falls within the range
specified by the tolerance) and control limits (the process is in control if the result
falls within the control limits).
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8.3.1 Inputs to Quality Control
.1 Work results. Work results (described in Section 4.2.3.1) include both process results and product results. Information about the planned or expected results (from the project plan) should be available along with information about the actual results.
.2 Quality management plan. The quality management plan is described in Section 8.1.3.1.
.3 Operational definitions. Operational definitions are described in Section 8.1.3.2.
.4 Checklists. Checklists are described in Section 8.1.3.3.
8.3.2 Tools and Techniques for Quality Control
.1 Inspection. Inspection includes activities such as measuring, examining, and testing undertaken to determine whether results conform to requirements. Inspections may be conducted at any level (e.g., the results of a single activity may be inspected or the final product of the project may be inspected). Inspections are variously called reviews, product reviews, audits, and walkthroughs; in some application areas, these terms have narrow and specific meanings.
.2 Control charts.
Control charts are a graphic display of the results, over time, of a
process. They are used to determine if the process is "in control" (e.g., are differences
in the results created by random variations, or are unusual events occurring
whose causes must be identified and corrected?). When a process is in control, the
process should not be adjusted. The process may be changed to provide
improvements, but it should not be adjusted when it is in control.
Control charts may be used to monitor any type of output variable. Although
used most frequently to track repetitive activities, such as manufactured lots, control
charts can also be used to monitor cost and schedule variances, volume and
frequency of scope changes, errors in project documents, or other management results
to help determine if the project management process is in control.
Figure 8-4 is a control chart of project schedule performance.
.3 Pareto diagrams. A Pareto diagram is a histogram, ordered by frequency of occurrence, that shows how many results were generated by type or category of identified cause (see Figure 8-5). Rank ordering is used to guide corrective action ___ the project team should take action to fix the problems that are causing the greatest number of defects first. Pareto diagrams are conceptually related to Pareto's Law, which holds that a relatively small number of causes will typically produce a large majority of the problems or defects. This is commonly referred to as the 80/20 principle, where 80 percent of the problems are due to 20 percent of the causes.
.4 Statistical sampling. Statistical sampling involves choosing part of a population of interest for inspection (e.g., selecting ten engineering drawings at random from a list of 75). Appropriate sampling can often reduce the cost of quality control. There is a substantial body of knowledge on statistical sampling; in some application areas, it is necessary for the project management team to be familiar with a variety of sampling techniques.
.5 Flowcharting. Flowcharting is described in Section 8.1.2.3. Flowcharting is used in quality control to help analyze how problems occur.
.6 Trend analysis. Trend analysis involves using mathematical techniques to forecast future outcomes based on historical results. Trend analysis is often used to monitor:
Technical performance—how many errors or
defects have been identified, how many remain uncorrected.
Cost and schedule performance—how many
activities per period were completed with significant variances.
8.3.3 Outputs from Quality Control
.1 Quality improvement. Quality improvement is described in Section 8.2.3.1.
.2 Acceptance decisions. The items inspected will be either accepted or rejected. Rejected items may require rework (described in Section 8.3.3.3).
.3 Rework. Rework is action taken to bring a defective or nonconforming item into compliance with requirements or specifications. Rework, especially unanticipated rework, is a frequent cause of project overruns in most application areas. The project team should make every reasonable effort to minimize rework.
.4 Completed checklists. See Section 8.1.3.3. When checklists are used, the completed checklists should become part of the project's records.
.5 Process adjustments. Process adjustments involve immediate corrective or preventive action as a result of quality control measurements. In some cases, the process adjustment may need to be handled according to procedures for integrated change control, as described in Section 4.3.
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