# Assignable Causes

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Suppose that the true fraction nonconforming p in the production process is known or is a specified standard value.

The the center line and control limits of the fraction nonconforming control chart would be as follows: When the process fraction nonconforming p is not known, then it must be estimated from observed data.

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.

Process control schemes that change the sample size and/or the sampling frequency depending on the position of the current sample value are called adaptive or variable sampling interval (or variable sample size, etc.) schemes.

Essentially, the ARL is the average number of points that must be plotted before a point indicates an out-of-control condition.There are always bound to be variances in a system, there are so many minute factors in any system.Environmental changes, wearing of tools, the accuracy of any human component, the frequency or lack of maintenance on necessary machinery, they all effect a system.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.Such special causes may be indicated or detected by control charts.Points of variance beyond control limits, or non-random patterns shown within the control limits are indicators of a special cause.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.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.