statistical quality control

statistical quality control

1. Types of statistical quality tools

   • 1. Design of experiment (DOE)
     • -development the key variables influencing the quality characteristics of product or process
   • 2. statistical process control (SPC)
     • - product control is a matter of defect detection and containment, and effects outgoing quality levels
     • - process control is a matter of defect prevention which affects both outgoing quality &productivity
     • - used to predict process performance and to achieve stability
     • types of variations
       • 1. Chance variation
         • - common variation/ natural variation
         • - impossible to fully eliminate (85% of quality problem)
         • -if only chance variation is present, output of process forms a distribution that is stable over time & is predictable
         • in statistical control
       • 2. assignment variation
         • as special variation , is easy to identify &eliminate (compare to Chance variation)
           • - 15% of quality problems
             • -if only assignable variation is present, output of process is not stable over time & is nor predictable
             • out of statistical control
     • Types of control charts
       • 1. variables
       • 2. attributes
     • Principles of control charts
       • - control charts are designed to detect deviation of process from a pre-determined normal status and flag early warning
         • - a process is considered normal if it is under statistical control, i.e. exhibits only chance variation
           • location and disersion
     • interpretation of control charts ( 8 rules)
     • Limitations of variable control chart
       • monitor only one quality characteristic per set of charts &several charts will be needed if > one
         • didn't tell us whether process is meeting specification tolerance (acceptable range as stated by customers), only indicate whether it is under statistical control
           • don't identify assignable causes & don't tell us how to eliminate effects of these causes
             • serve as statistically-based warning devices for process problems
   • 4. acceptance sampling

2. 3. process capability study

   • - reflect the ability of a process to satisfy tolerance specification

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