Examining the Use of Control Charts for Projects
Control Charts are graphs that are used to analyse and communicate the variability of a process activity over time. Control charts help to show the potential capacity of the process and also suggest the range of variability in the process.
This range of variability can assist project Manager in determining if the variance is caused by common or assignable sources. If the process variability fluctuates around the average of statistical mean, the process shows very little variability and is said to be stable.
The components of a control chart include the process mean, Upper Control Limit(UCL) and the Lower Control Limit (LCL). The process mean is determined by taking samples from the actual process and calculating the statistical mean. An additional sample is taken and tested, they are evaluated in terms of standard deviation from the process mean. For most organisations, and projects, the UCL will be three standard deviations above the mean, while the LCL will be three standard deviations below the mean.
Control Charts have a measurement that indicates instability because they each have measurement that exceeds the range between the UCL and LCL. Analysis of the first chart shows that there are more than seven consecutive points above the means. In the second chart more than seven consecutive points below the mean. This seven-point variance is called “The run of seven”.
Seven run rules…
Run rules are used to indicate situations that are out of statistical control. When seven or more consecutive points lie on the side of the mean, it indicates that there should be a shift in the mean.
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