Higher National Certificate in Engineering Unit 36 Lesson 2 – Special Cause v Common Cause

Learning Outcome 1.1 LO 1.2: Describe the significance of natural and assignable causes of variation

What causes Variability? The basis of the theory of statistical process control is differentiation of the causes of variability during the operation of any process. Certain variations belong to the category of of chance or random variations – about which little can be done, other than to revise the process.

What causes Variability? This type of variation is the sum of the multitude of effects of a complex interaction of ‘random’ (common) causes many of which are slight. When random variations alone exist – atmospheric pressure or temperature; electrical or humidity fluctuations, passing traffic or equipment fluctuations; changes in the operator’s physical or emotional state - it will not be possible to trace their causes

What causes Variability? When only common causes of variation are present in a process, the process is considered to be ‘stable’ in ‘statistical control’ or just ‘in control’. There is also variation in any test equipment and inspection / checking procedures, whether used to measure a physical dimension, an electronic or chemical characteristic or property of an information system. These variations in checking and testing contribute to the overall process variability.

What causes Variability? Causes of variation which are relatively large in magnitude and readily identified are classified as ‘assignable’ or ‘special cause’ variation. When special causes of variation are present, variation will be excessive and the process in classified as ‘unstable’, or ‘out-of-control’.

In-control or Out-of-control? Is the process stable (in-control)? –Are there any any special causes of variation or is the process variability due to common causes only? What is the extent of the process variability? –What is the natural capability of the process when only common causes of variation are present?

Variability Variability cannot be ignored. The simple fact that a measurement, test or analytical method is used to generate data introduces variability. This must be taken into account and appropriate charts of data used to control processes instead of reacting to individual results.

Variability It is often found that range charts (R-charts) are in control and indicate an inherently capable process. The saw tooth appearance of the mean chart (x-bar Charts) however shows rapid alteration in the mean of a process.

Variability When a process is found to be out of control the first reaction must be to investigate the assignable (special) cause of variability. This may require in some cases the charting of process parameters rather than the product parameters which appear in the specification. For example it may be that viscosity of a chemical product is directly affected by the pressure in the reactor vessel which in turn may be affected by the reactor temperature.

Variability A control chart for pressure with recorded changes in temperature may be the first step in breaking into the complexity of the relationship involved. The important point is to ensure that that all adjustments to the process are recorded anmd the relevant data charted.

Variability The can be no compromise on processes which are shown to be not in control. The simple device of changing the charting method and / or the control limits will not bring the process into control. A proper process investigation must take place.

Variability There are numerous potential special causes for processes being out of control. It is extremely difficult even dangerous to try to find an association between types of causes and patterns shown on control charts. There are clearly many causes which could give rise to different patterns in different industries and conditions.

Special Causes People: –Fatigue or illness –Lack of training / novices –Unsupervised –Unaware –Attitudes / motivation –Changes / improvement –Rotation of shifts

Special Causes Plant / equipment –Rotation of machines –Differences in test or measuring devices –Scheduled preventative maintenance –Lack of maintenance –Badly designed equipment –Worn equipment –Gradual deterioration of plant /equipment

Special Causes Processes /procedures –Unsuitable techniques of operation or test –Untried / new processes –Changes in methods, inspection or check

Special Causes Materials –Merging or mixing of batches, parts, components, subassemblies, intermediates, etc. –Accumulation of waste products –Homogeneity –Changes in supplier / material

Special Causes Environment –Gradual deterioration in conditions –Temperature changes –Humidity –Noise –Dusty atmospheres