If you know which factors are important for quality and you control them within the acceptable variation, you achieve the wanted product quality.
To improve quality, it is important to consider all factors that can be significant for product quality. Start with process mapping to identify variables that can affect product quality.
All processes are subject to variation, which may be classified as random / noise (common cause) or assignable cause variation (special cause).
Common cause variation is found in all processes and exhibits random yet stable variation that is predictable within calculated process control limits. Common cause variation is synonymous with natural variation, expected variation, or random variation.
Special cause variation, on the other hand, is unpredictable, sporadic, or unstable variation. It is the result of a specific assignable cause, for example Machine (Equipment), Man (Operator), Material, Method, Milieu (environment) & Measurement (referred to as 6M in a Fishbone).
Humidity is an example of a specific assignable cause (Milieu) that can be controlled. The challenge of quality assurance is to identify the critical factors and control that the variation of them are within allowed tolerances.
Six Sigma involves statistical tool to understand variation and identify critical factors. It is important to know the critical factors of your processes to avoid spending time controlling non-important factors. Control of critical factors gives predictable results and satisfied customers if the quality is within the customer specifications. There are examples of companies that have canceled the launch of new product because they have not identified all critical factors for quality. The variation of product quality was unacceptable for the customer. If critical factors are unknown it's difficult to reduce variation.
What are the critical factors of your processes? How do you control their variation?
Tools to improve quality
Here are some tools that can help you improve quality:
- Statistical Process Control and capability reveal whether normal or special cause variation is causing quality issues. These types of variation have different improvement strategies.
- Process mapping identifies variables that contribute to variation of product quality.
- Graphical analysis can prove or disprove root causes
- Hypothesis testing can prove or disprove root causes when graphical analyzes are not clear
- Correlation and design of experiment can help you determine optimal factor values.
- Statistical Process Control (Control Chart) monitors variation of critical variables (proven root causes) and ensures correct quality
These tools are used within Lean Six Sigma's problem-solving method DMAIC: Define (understand problem and set goals) - Measure (map current situation) - Analyze (prove root causes) - Improve (solve root causes) - Control (create lasting results).