Process Capability And Control Charts

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In this presentation topic on Process Capability And Control Charts, we will look into the basics of Process Capability, Types of control Charts, Development and maintenance of control charts, Interpretation of control charts with help of a practice problem.

Proper strategy for quality control is to Control the process rather than sorting inspection of the product. Variability is a norm but it should be minimized. The two causes of variation are common causes that are inherent in the process over time and special causes that are not part of the process all the time but arise because of specific circumstances. Statistics can help identifying the presences of special causes.

Process variation

Process variation is caused by Inherent conflict between two points. Here variation is inevitable. The production is most economical when output is of uniform quality. the source of Variation are Man, Material, Machine, Mood, Environment etc.

Need for process capability study stems from the fact that, a manager is interested in knowing whether the process is capable of producing the products of desired quality, What proportion of the products produced by process are expected to be accepted/rejected and what changes are needed to improve the capability of the process or to reduce the rejection rate. Process capability study provide answers to all of the above questions.

Process capability

It refers to the performance of the process when it is operating in statistical control. It is a measure of the uniformity of a process that makes a product. A common measure of process capability is 6-sigma or spread . It makes sense only when the process is in statistical control.

By Definition, "It is performance under chronic conditions in which sporadic variation do not exist".

Process capability analysis

It is an engineering study to estimate process capability. It may involve estimating the process mean and standard deviation. It might also involve estimating the proportion of nonconforming items.


It help us by predicting how well the process will hold the tolerances. It assists us in product and process development and specifying performance requirements for new equipment. Assists in vendor selection. It help us to reduce variability and reduces total cost by lowering internal and internal failure costs.

Specification and control limits

There is no relation between specification and control limits.

In case of specification Limits (or tolerance limits)

  • Limits that define the conformance boundary for an individual unit
  • Provided by the designer or the customer for the products for its functioning

In case of control Limits (or natural tolerance limits)

  • Outcome of the process identifies process variability¬†
  • Does not apply to an individual units
Control Limits

Stable and capable process

Stable process is whose output is influenced only by chance causes and not by special causes. This does not imply that the output will be as per specifications. Capable process is one that is expected to produce output conforming to the specifications when the process is in control.

Capable Incapable Process

Control Chart

A control chart is a graphical tool for monitoring a process. It indicates presence/absence of special cause(s) of variation in the process. It is an on-line quality control tool. Different type of control charts are Variable control charts (X-bar chart, R chart) and Attribute control charts (p chart,c chart).

To create control charts, we need to partition the historical data. One for the construction and other for reflecting on the performance. It uses data to identify 3 sigma limit. To craw the control charts, use Y axis for variable measurement, X axis for sequence of samples. Try to plot the most recent average.

If we compare variable and attribute charts, we can see that variables provide more information than attributes. Cost of inspection for obtaining variable data is normally higher than for attributes. Attribute charts are more useful at plant level while variable charts at the machine or operator level.

Control charts helps us distinguish special from common causes of variation to guide management action. It can be used by operators for on-line control of a process. It provides predictability for quality and cost. It allow the process to achieve Higher quality, Lower unit cost and Higher effective capacity.

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