Control Charts is a running record of the process performance. It is a recod of results of periodic sampling inspections.
A chart becomes a control chart when it has control limits based on inherent process variation. Process control limits are boundaries on a control chart within which the process can operate to a standard. These limits are based on natural variation of the process without the influence of assignable causes. each time the job is checked, the results are compared with the control limits. If the results are within the control limits, then the process is to be left alone. But if a point on a control chart falls outside the control limits, or any other indications of an out of control process, it indicates that there is some change which is happened and the process in no lnger operating normally.
In other words, Control Limits are warning singnals that tell us
1. When to take action
2. When to leave the process alone.
Taking action on a process operating within control limits is not only eneconomcal but may also increase the variation.
There are two general types of Control Charts
1. Variables Chart – This type of chart is used where a dimentsion of a charecteristin is meaeasured and the result is a value.
Popularly used Variables charts are
X-Bar – R Charts
X-Bar – S – Charts
2. Attributes Chart. – This type of chart is used where a product quality is assessed by sensory means or the data is in terms of count of defectives of count of defects.
Teh popularly used Attributes Charts are
In addition to above there are some adapations to the control charts which are a combination of the above. These are called Special Contorl Charts which will be discussed later.