Types of Variation

Common causes are inherent and predictable types of variation in the process and affect, in some extent, all products/inputs to the process [1]. Processes subject to common cause variation will exhibit stable and consistent patterns of variation over time as shown in Figure 2. Examples of this would include:-

• Working Environment (e.g. light, temperature, air pressure...etc)
• Machinery Characteristics (e.g. working temperature, accuracy, age)
• Material Properties (e.g. hardness, colour, smoothness)
• Operator Variability (e.g. use of machinery, part handling...etc)

Figure 2: An example of a process output
with common cause variation

Special Causes are momentary in nature and are not inherent in the process. They will affect only some products of the process and the overall output. Processes subject to special causes will exhibit unpredictable patterns of variation, which change overtime.

Examples of Special Causes may include:

• Poorly trained operators (e.g. Parts assembled incorrectly, new trainee etc)
• Machine breakdown (e.g. tool wear, mechanical failure, machine settings)
• Defective material (e.g. non-compliant material)
• Environmental changes ( e.g. tornado, rain storm, sudden temperature drop)

Special Causes must always be investigated as soon as possible after its presence have been highlighted as it may have an adverse effect on the process and its output as shown in the example of Figure 3.

Figure 3: An example of a process output containing special cause variation