# Assignable Causes

For the chart with three-sigma limits, p = 0.0027 is the probability that a single point falls outside the limits when the process is in control.Therefore, the average run length of the chart when the process is in control (called ARLIt is also occasionally convenient to express the performance of the control chart in terms of its average time to signal (ATS).Such special causes may be indicated or detected by control charts.

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If the process observations are uncorrelated, then for any Shewhart control chart, the ARL can be calculated easily from [image]where p is the probability that any point exceeds the control limits.

This equation can be used to evaluate the performance of the control chart.

In this case R incorrectly measures the variability between the different underlying distributions, in addition to the chance cause variation that it is intended to measure.

Many quality characteristics cannot be conveniently represented numerically.

Consequently, the routine and attentive use of control charts will identify assignable causes.

If these causes can be eliminated from the process, variability will be reduced and the process will be improved.A special cause refers to a system used in project management.A special cause, also called an assignable cause, is any factor or factors which may affect a system either in progress or outcome.A Special cause however, is not predictable or stable.That is the problem; as such it may only be assigned by project management personal as a defect in the system.If samples are taken at fixed intervals of time that are h hours apart, then Those rules apply to one side of the center line at a time.Therefore, a point above the upper warning limit followed immediately by a point below the lower warning limit would not signal an out-of-control alarm.Typically these crate variations in the function of a system or its outcome in a stable, repeatable, and predictable pattern over time.These are called common causes and may be taken into account by project management.In the framework of statistical quality control, this natural variability is often called a “stable system of chance causes.” We refer to these sources of variability that are not part of the chance cause pattern as assignable causes of variation.A process that is operating in the presence of assignable causes is said to be an out-of-control process.2.