Variation Meaning | Process Variation | Common causes Vs Special causes

Variation is the basic principle of Statistical Process Control (SPC).

One problem with mother nature is  “Variability”  i.e. No two things are alike and Everything is different.

Variability

• No two things are alike.
• The product we produce change continuously.
• Variation is law of nature and exists in all processes.
• Variation in a product and process can be always measured.

PROCESS VARIATION TYPES

There are two types of process variation present in the manufacturing process :

1. Common causes : Also called as Chance causes (natural).

2. Special Causes : Also called as Assignable cause (un natural).

Common cause (Chance causes) :

• Common causes are always present on the process.
• Sources of variations present in process is stable over the time.
• Process is said to be under statistical control.
• Voice of process is predictable.
• Output of process is predictable.
• Local action required to control process variation.

A control chart having a Common cause will have the following characteristics:

• Very few points will be near to the control limits.
• Most of the points will lie near to the central line.

Special Causes (Assignable causes):

• Special causes are not always acting on the process.
• Process is not under Statistical control.
• Process output is unpredictable.
• Process is not stable over time.
• Erratic fluctuations and Shift occurred in process.
• Management action required to control process variation.

Type of Special Causes :

1. Extreme Variations : Recognized by the points falling outside the Upper and Lower control limits.
2. Erratic Fluctuations : Erratic fluctuation is characterized by ups and downs. This may be due to single cause  or a group of causes affecting the process.
3. Shifts : When a series of consecutive points fall above or below the centre line on the control chart.
4. Indication of Trend : When the consecutive points on control chart tend to move steadily either towards Upper Control Limits (UCL) or Lower Control Limit (LCL),  then it can be assumed that process is indicating a ‘Trend’.

Inherent Process Variation

Inherent Process Variation is the portion of Process variation due to Common causes only and is determined by Process Standard Deviation (σ).

Total  Process Variation

It is the variation due to both Common causes (natural) and Special Causes (un natural) causes.