Is Process Stable The Quality Improvement Model Define

Is Process Stable ? The Quality Improvement Model Define Process Select Measures Collect & Interpret Data Is Process Stable? Is Process Stable ? Purpose: Determine the stability of key measures of the product. No Investigate & Fix Special Causes Yes Improve Process Capability No Is Process Capable ? Yes Use SPC to Maintain Current Process 6 -1

Is Process Stable ? Types of Variation Common Causes that are inherent in the process over time, and affect all outcomes of the process. l Ever-present l Create small, random fluctuations in the process l Lots of them l The sum of their effects creates the expected variability l Predictable Run Chart Quality Characteristic Time 6 -2

Is Process Stable ? Types of Variation Special Causes that are not present in the process all the time, but arise because of specific circumstances. l Not always present in the process l Can create large process disturbances, or sustained shifts l Relatively few in number l Pull the process beyond the expected level of variability l Unpredictable Run Chart Quality Characteristic Time Control charts help identify the presence of special causes. 6 -3

Is Process Stable ? Control Chart Components Run chart of the data l Center Line (CL) l § l UCL A line at the average of the data or target of the process Upper Control Limit (UCL) § l Control Chart A line at the upper limit of expected variability Lower Control Limit (LCL) § CL LCL 2 4 6 8 101214161820 2224 26283032 Run Order A line at the lower limit of expected variability The control limits are based on data collected from the process. 6 -4

Is Process Stable ? Rules for Separating Common & Special Causes Two commonly used signals of special causes are: Rule 1: Any point above the Upper Control Limit (UCL) or below the Lower Control Limit (LCL) Rule 2: 8 points in a row on the same side of the center line (CL) Note: Additional rules do exist. 6 -5

Process Stability Is Process Stable ? Stable Process A process in which the key measures of the output from the process show no signs of special causes. Variation is a result of common causes only. Unstable Process A process in which the key measures of the output from the process show signs of special causes in addition to common causes. Variation is a result of both common and special causes. 6 -6

Process Stability Is Process Stable ? 6 -7

A Stable Process UCL LCL Is Process Stable ? • • • • • • • • • • • • • • • • • • • • • Common Causes Alone Are At Work: • Behaves in a Random Manner • No Cycles • No Runs • No Trends • No Shifts • No Defined Patterns 6 -8

Un-Stable Process UCL LCL Is Process Stable ? • • • • • • • • • • • • • • • • • • • • • Assignable Cause(s) Are Present In Addition To Common Cause Variation Look For: • Points Outside the Control Limits • Shifts • Cycles • Runs • Trends 6 -9

Is Process Stable ? Advantages of Stable Processes Are: 6 -10

Is Process Stable ? Polymer Manufacturing Data Control Chart b* Histogram 5 4 LS US 3 UCL=2. 2 2 Avg=1. 4 1 LCL=0. 5 0 0 1 2 3 4 b* 5 20 40 60 80 Sample 100 120 140 Note: b* is a measure of yellowness Histogram does not show whether the process is stable! 6 -11

Histograms & Control Charts Histograms Control Charts Plot past data l Cannot tell if process is stable l Only useful for prediction if the process is stable l l Is Process Stable ? Real-time evaluation l Help identify presence of special causes l Assess past and present stability of process 6 -12

Is Process Stable ? Pump Maintenance Data 20 18 16 Number of Failures 14 12 UCL=11. 4 10 8 6 Avg=4. 8 4 2 LCL=None 0 2 4 6 8 10 12 14 16 18 20 22 24 Week Are there any signals of special causes? Circle them. 6 -13