Control Charts
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Control charts (also called Shewhart charts) are a powerful statistical quality control tool used for online process monitoring. Control charts detect assignable causes of variation by monitoring the process for points outside the natural limits called control limits. This ensures variations are kept within specification limits, delivering more consistent quality. There are different types of control charts for variables and attributes. Control charts must be acted on if points fall outside control limits or show non-random patterns, indicating the presence of assignable causes that need investigation and elimination.
Control Charts
- 1. CONTROL CHARTS (SHEWHART CHARTS) A presentation by THE SOCIETY OF STATISTICAL QUALITY CONTROL ENGINEERS BHOPAL
- 2. WHAT IS CONTROL CHART A powerful SQC tool for on-line process control
- 3. HOW CONTROL CHART WORKS Control Chart detects assignable causes ( also called special causes ) in the process. Once the assignable causes are eliminated, the process runs under the influence of chance causes alone and the process is said to be under statistical control. Variations are, then, controlled within natural limits called “control limits” which are generally narrower than the specification limits. CONTROL CHARTS ADVANTAGES (1) Control chart prevents defects in the process (2) Control chart keeps the variations to a minimum, generally narrower than the specification limits. The result : process delivers better and consistent quality products.
- 4. UNDERSTANDING CHANCE CAUSES AND ASSIGNABLE CAUSES OF VARIATION Variation is inevitable in any process /operation/component Variations occur due to the following two causes: (i) Chance causes (ii) Assignable causes
- 5. VARIATION DUE TO CHANCE CAUSES (Also called natural variation or inherent variation) • These variations are due to a large number of factors which are either uncontrollable or uneconomical to control. Examples of such factors are: temperature, humidity variations, variation in other environmental conditions, slight variations in raw material etc. • These variations are unavoidable. It is not possible to enumerate each and every chance cause and eliminate it. • The factors causing chance variations are many, but the effect of a single cause is very less. Variation due to chance cause is the sum total of variations due to all chance causes – known or unknown. • Variations due to chance causes follow certain standard probability distributions like Normal distribution in case of variable type of data and binomial/poisson distribution in case of attribute type of data.
- 6. VARIATION DUE TO ASSIGNABLE CAUSES (Also called variation due to special causes/ unnatural causes) • These variations are due to just one or few individual causes. • Even a single assignable cause results in a large variation. • Assignable causes being few, they can be detected and eliminated. • Examples of assignable causes: variations due to defective raw material, faulty setup or untrained operator. • Variations due to assignable causes do not follow any probability distribution and hence they can easily be “singled out” from chance variations.
- 7. CONTROL CHART TYPES (1) Control charts for VARIABLES (2) Control charts for ATTRIBUTES
- 10. CONTROL CHART FOR ATTRIBUTES CONTROL CHART TYPE QUALITY CHARACTERISTIC p – chart Proportion defective or percent defective np – chart Number of defectives c – chart Number of defects in a defined “unit” u – chart Number of defects per piece
- 16. WHEN TO ACT ON A CONTROL CHART The requirements in a control chart are: (i) All the points should lie within control limits (ii) The points should be randomly distributed. (i.e. the points should not depict any rising/falling trend, cyclic pattern, clustering of points above or below central line, shift of process average etc.) Violation of either of the above two conditions indicates presence of assignable cause in the process which must be investigated and eliminated. In case of p-chart/np-chart/c-chart/u-chart, if a point goes below the lower control limit, it should not be ignored thinking that it is an improvement. The cause of improvement must be investigated and if it is true, the same should be implemented in the process.
- 18. RUN CHART
- 19. WHAT IS A RUN CHART Run chart is a process control chart much simpler but less powerful than control chart. Run charts generally precede control charts. In a run chart individual observations (not averages) are plotted on time scale in order of production. The chart shows central line (nominal value) and specification limits. WHEN TO ACT ON A RUN CHART The requirements in a run chart are: (i) All the points should lie within specification limits (ii) The points should be randomly distributed. Violation of either of the above two conditions calls for investigation and corrective action.