According to the PMBOKGuide, specifically within the Control Quality process, a Control Chart is a graphic display of process data over time and against established control limits.
* Specification Limits: These are based on the requirements of the agreement (contract) or the customer ' s needs. They represent the maximum and minimum values allowed. If a product or service falls outside these limits, it is considered nonconforming (a defect).
* Control Limits vs. Specification Limits:
* Control Limits (Upper and Lower Control Limits - UCL/LCL) are calculated statistically (usually
$\pm3$ sigma) and show the natural variation of the process. They determine if the process is " in control. "
* Specification Limits (Upper and Lower Specification Limits - USL/LSL) are provided by the customer or contract. A process can be " in control " (statistically stable) but still " out of spec " if the control limits fall outside the specification limits.
* Purpose: The control chart allows the project manager to identify when a process is behaving unpredictably (out of control) or when it is in danger of violating the contractual specification limits.
Comparison with other options:
* B. Flowchart: This is a graphical representation of a process showing how various elements of a system relate. It is used to identify where quality problems might occur but does not track data against specification limits.
* C. Checksheet: Also known as a tally sheet, this is used to organize facts in a manner that will facilitate the effective collection of useful data about a potential quality problem. It is a data collection tool, not an analytical chart for limits.
* D. Pareto diagram: This is a specific type of vertical bar chart used to identify the vital few sources that are responsible for causing most of a problem ' s effects. It follows the 80/20 rule and does not incorporate upper or lower specification limits.