# Statistical Process Control - PowerPoint PPT Presentation

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Statistical Process Control. (SPC). What is Quality?. Fitness for use Conformance to the standard. Quality Improvement. Quality improvement processes that involve statistical method; Incoming Quality Control Statistical Process Control Outgoing Quality Control. Incoming & Outgoing QC.

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Statistical Process Control

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## Statistical Process Control

(SPC)

### What is Quality?

• Fitness for use

• Conformance to the standard

### Quality Improvement

Quality improvement processes that involve statistical method;

• Incoming Quality Control

• Statistical Process Control

• Outgoing Quality Control

### Incoming & Outgoing QC

Acceptance Sampling

• Lot by lot sampling plan for attributes

• Acceptance sampling by variables

### Lot by Lot Sampling Plan For Attributes

Types Of Sampling Plan

• Single Sampling Plan

• Double Sampling Plan

• Multiple Sampling Plan

• Sequential Sampling Plan

MIL STD 105E (ISO 2859)

### Acceptance Sampling By Variables

Types Of Sampling Plan

• Plan that control the lot/process fraction defective/nonconforming.

• Plan that control the lot/process parameter

MIL STD 414 Tables

### Statistical Pocess Control

Chance and Assingable Cause Of Quality

Statistical basis for control chart

• Basic Principles

• Choice Of Control Limit

• Sample Size & Sampling Frequency

• Subgroups

• Analysis Of Patterns On Control Charts

### Control Charts For Variables (Univariate)

• Statistical basis of the charts

• Development and use of the charts

• Interpretation of the charts

• The operating characteristics function

• Average Run Length (ARL) for the mean chart

• Constuction & Operation of the charts

• Control charts with variable sample size

### Control Charts For The Attributes

Control Charts for the fraction non-conforming

• Development & Operation

• Variable sample size

• OC and ARL

Control Charts for non-conformities

• Procedure with constant sample size

• Procedure with variable sample size

• OC and ARL

### Process Capability Analysis

• Using histogram

• Using probality plots

• Process Capability Ratio (PCR)

• Cp

• PCR for an of center process

• Normality and PCR

• Confidence Interval & Test on PCR

• PCR using control charts

### Chance and Assingable Cause Of Quality

• 2 types of variation

• Natural variability (chance)

• Assignable causes

• Process with assignable causes is said to be out of control.

### Basic Principles

• Control charts consist of

• Center line

• Upper control limit

• Lower control limit

These limits is chosen so that when the process is in control, almost all the sample points will fall within the control limits.

### Choice Of Control Limit

2 types of control limits

• Three-sigma limits.

• The distance between CL and the UCL/LCL is 3 sigma.

• The 0.001 probability limits chart (use 3.09 sigma).

• The distance between CL and the UCL/LCL is 3.09 sigma.

Note:-

3-sigma limits popular in US.

0.001 prob. limits popular in UK & Western Europe.

### Sample Size & Sampling Frequency

• Larger samples easier to detect.

• Use large sample if the shift of interest is small and small sample if the shift of interest is large.

• Frequent sampling is better.

Current practice favour small but more frequent samples.

### Analysis Of Patterns On Control Charts

Process out of control if

• One or more points fall outside the control limits.

• Points exhibit some non-random pattern.

• Exhibit a cyclic behaviour.

### Pattern recognition

Process is out of control if

• One point plots outside the control limit.

• Two out of three consecutive points plot beyond the two sigma warning limits.

• Four out of five consecutive points plot at a distance of the one sigma or beyond from the center line.

• Eight consecutive points plot on one side of the center line.

### Control Charts For Attributes

• Control Chart For Fraction Nonconforming

• Control Chart For Nonconformities

### Process Capability

Calculated using

• Process capability ratio (PCR), Cp.

• Probability ;

*need to know process std deviation

### Process standard deviation

Process standard deviation is calculated by

*use to estimate process capability

### Process Capability Ratio (PCR), Cp

And is estimated by

### Interpretation

What does it mean if

• Cp < 1

• Cp = 1

• Cp > 1