This document provides an overview of statistical process control (SPC). SPC uses statistical techniques to monitor processes and detect changes, helping to prevent defects and drive continuous improvement. It aims to identify problems in production as early as possible through analysis of process capability. Control charts are a key tool in SPC, monitoring the average and variation of a process over time through control limits. Being in statistical control means a process's measurements vary randomly within control limits in a predictable way. Patterns outside the limits indicate the process is out of control due to assignable causes that need correction. Benefits of SPC and control charts include reduced scrap, preventing unnecessary adjustments, and providing diagnostic information.

1. Statistical Process
Control (S.P.C.)
By Lorraine Fraser

2. • A control system which uses statistical techniques for
knowing changes in the process, an effective method in
preventing defects and in helping continuous improvement.
Introduction to
Statistical Process Control (S.P.C.)
• Detects as quickly as possible a significant departure from the
norm. An assignable cause is something that can be corrected
at the machine level, though requires an intimate understanding
of the manufacturing process.
• S.P.C is a statistical analysis of the predictability and capability
of a process to give a uniform product.
16
14
12
10
8
6
4
2
0 36.31 36.32 36.33 36.34 36.35 36.36 36.37 36.38 36.39

3. What does S.P.C. mean?
• Statistical - tools are used to make predictions on
performance with a high degree of accuracy.
• Process - the process involves people, machines, materials,
methods, management and environment working together to
produce an output, such as an end product.
• Control - This is a control system which uses statistical
techniques for knowing changes in the process. It is an
effective method in preventing defects and helps continuous
quality improvement.

4. The Aim of S.P.C.
- Detection Strategy
 Historically a detection type system focused on
identification of problems after production, by 100%
inspection or by customer complaints. The drawbacks to
this process are higher costs (rework, inspection), and
repetitive problems.
Benefits of S.P.C.
• Monitors performance, preventing defects.
• Common language for discussing process performance.
• Reduced cost of inspection and defects with a stable process.
• Is a tool for prevention vs. detection.
 S.P.C. focuses on in-process production and identification of
problems through analysis of process capability. It is a
future oriented strategy.

5. • When dealing with variable data (measurements), both
the location and spread of the frequency distribution must
be monitored to maintain process control, capability and
centring.
• We turn to control charts to monitor process average and
spread based on average and range of small samples.
• To monitor the location and spread of the process, control
limits will be established to define the expected amount of
variation of both the sample average and sample range.
• If the process is initially controlled, capable and centred,
and the sample data confirm that the process continues to
operate within this expected variation, then the resulting
product or parts will reliably fall within their specification
limits.
• Control charts are real time control tools.
Process Control Charts

6.  Control limits are not specification limits. Specifications
reflect customer requirements, while control limits reflect
the historical performance of the process.
 Out of control - assignable causes.
 In control - random causes
 Subgroup size – 4 or 5 is most common for economy and
confidence
Process Control Charts

7.  A control chart is a graph used to study how a process
changes over time.
 Control charts for variable data are used in pairs. The top
chart monitors the average, and the bottom chart monitors
the range, or the width of the distribution.
 Variable data (measurements) are measured on a
continuous scale.
Process Control Charts

8. Example of a Control Chart
UCL
USL
LCL
LSL
Target

9. Process that is in ‘Statistical Control’
Process measurements vary randomly within the control
limits and the variation is predictable over time.

10. Out of Control Signals on a Control
Chart

11. Self-test (9 of 9)Benefits of Control Charts
 Reduce scrap and re-work by the systematic elimination
of assignable causes
 Prevent unnecessary adjustments.
 Provide diagnostic information from the shape of the non
random patterns.
 Find out what the process can do.
 Provide immediate visual feedback.
 Helps you recognize and understand variability and how
to control it.
 Identify ‘special causes’ of variation and change in
performance
 Keep you from fixing a process that is varying randomly
within control limits. If you want to improve it you have to
objectively identify and eliminate the root cause of the
process variation.

12. Self-test (9 of 9)
Questions?