1. Higher National Certificate in Engineering
Unit 36 – LO 1.2

2. Before starting this LO please teach LO1.3 first

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

4. What causes Variability?
The basis of the theory of statistical process control (SPC) is the differentiation of the causes of variability during the operation of any process.
When differentiating causes we broadly put them in one of two classes: natural (or common) and assignable (or special) causes.

5. What causes Variability?
Natural (common) causes of variation belong to the category of chance or random variations – about which little can be done, (other than to revise the process).
Natural variation is the sum of the multitude of effects of a complex interaction of ‘random’ (common) causes, many of which are slight.

6. What causes Variability?
Natural random variations might be due to for example, slight differences in atmospheric pressure or temperature or electrical current flow or humidity fluctuations, passing traffic or equipment fluctuations, or even changes in the operator’s physical or emotional state.
Due to the sight variations in these causes of natural variation it will not be possible measure and hence trace their causes.

7. What causes Variability?
We have to accept that these causes are omnipresent and that there is not a lot we can do about them accept possibly to change the process.
When only common (natural) causes of variation are present in a process, the process is considered to be ‘stable’ i.e. it is ‘in statistical control’ or just as it is often phrased ‘in control’.

8. The Normal Distributionx _
x ± σ -
2x ± σ -
3x ± σ -
Range

9. The Normal Distribution
Note:
68.3 % of the population lie within ± 1 s.d. of the mean i.e. x ± σ
95.4 % within x ± 2σ
99.7 % within x ± 3σ - - -

10. What causes Variability?
It should be noted that there is also variation in the test equipment used to take measurements (i.e. measuring lengths of wire with a ruler) and in inspection / checking procedures (checking yellow coloured circles), whether used to measure say 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 and are sometimes termed ‘error’.

11. What causes Variability?
Another type of cause of variation can ‘show’ its presence in a process, as it is more easily identified than common cause. This is because it is relatively large (large that is in comparison to common causes) in magnitude.
This type of cause is classified as assignable (special) cause variation.
When special causes of variation are present, variation will be excessive and the process in classified as unstable (out-of-control).

12. In-control or Out-of-control?
Is the process stable (i.e is it in-or-out of control)?
This is really asking, ‘Are there any any special causes of variation prevalent in the process or is the process variability due to common causes only?’
If the process is stable then what is the extent of (i.e. how ‘big’ is) the process variability?
Being stable means that there is no special cause present, thus with only common cause variation affecting the process what is the natural capability of the process?

13. Points regarding the Variability of a Process
Variability cannot be ignored.
The simple fact is that taking a measurement, performing a test or undertaking an analytical method to generate data, will introduce variability.
Variability must be taken into account and appropriate charts used to record data to enable a company to interpret what a process is ‘doing’ and thereby control it, rather than reacting to an individual result or piece of data.

14. Now refer back to LO1.3