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How to use the seven tools of quality

Tools for identifying problems / collecting dataCheck sheetsScatter diagramsStatistical process control (SPC) chart

Tools to organize the dataPareto chartsHistogram

Tools for generating ideasCause-and-effect diagramsFlowcharts

DMAIC Process

• Define• Identify customers and their priorities• Identify and define a suitable project • Identify CTQs (critical-to-quality characteristics)

• Measure• Determine how to measure the process and how it is performing• Identify key internal processes that influence CTQs• Measure current defects

DMAIC Process

• Analyze• Determine likely causes of defects• Understand why defects are generated by identifying key variables that cause

process variation

• Improve• Identify means to remove causes of defects• Confirm key variables and quantify their effects on CTQs

DMAIC Process

• Improve• Identify the maximum acceptable ranges of the key variables and a system for

measuring deviations of the variables• Modify the process to stay within the acceptable range

• Control• Determine how to maintain improvements• Put tools in place to ensure that key variables remain within acceptable

ranges under the modified process

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Linking the seven tools to the DMAIC quality cycle

Defineflowcharts (to understand process – baseline knowledge)

Measurechecksheets (to collect data)scatter diagrams (to collect data / look for patterns)

Analyseflowcharts (looking for steps where mistakes occur)cause & effect diagrams (to suggest possible sources of error)histograms (to organise / understand data)pareto charts (ranking errors most important to least important)

Improve?????

Controlrun and control charts (to monitor processes)

Cost of Quality Measurement

• Cost of quality: Costs associated with avoiding poor quality or those incurred as a result of poor quality

• Applications• Better communication between operations managers and senior-level

managers• Identify and justify major improvement opportunities• Evaluate the importance of quality and improvement in operations

Categories of Quality Costs

• Prevention costs: Expended to keep nonconforming goods and services from being made and reaching the customer

• Quality planning costs• Process-control costs• Information-systems costs• Training and general management costs

Categories of Quality Costs

• Appraisal costs: Expended on ascertaining quality levels through measurement and analysis of data to detect and correct problems

• Test and inspection costs• Instrument maintenance costs• Process-measurement and process-control costs

Categories of Quality Costs

• Internal failure costs: Incurred as a result of unsatisfactory quality that is found before the delivery of a good or service to the customer

• Scrap and rework costs• Costs of corrective action• Downgrading costs• Process failures

Categories of Quality Costs

• External failure costs: Incurred after poor-quality goods or services reach the customer

• Costs due to customer complaints and returns• Goods and services recall costs and warranty and service guarantee claims• Product-liability costs

Internal Failure Appraisal Prevention External Failure0

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Quality Costs for a manufacturer

Costs Areas

$ (0

00,0

00s)

All production processes have some variation

Time / number of samples

Data range

UCL

LCL

mean

Resulting normal distribution

Mean and Range Charts

(a)

These sampling distributions result in the charts below

(Sampling mean is shifting upward, but range is consistent)

R-chart(R-chart does not detect change in mean)

UCL

LCL

x-chart(x-chart detects shift in central tendency)

UCL

LCL

Mean and Range Charts

R-chart(R-chart detects increase in dispersion)

UCL

LCL

(b)

These sampling distributions result in the charts below

(Sampling mean is constant, but dispersion is increasing)

x-chart(x-chart indicates no change in central tendency)

UCL

LCL

Patterns in Control Charts

Normal behavior. Process is “in control.”

Upper control limit

Target

Lower control limit

Figure S6.7

Patterns in Control Charts

One plot out above (or below). Investigate for cause. Process is “out of control.”

Upper control limit

Target

Lower control limit

Figure S6.7

Patterns in Control Charts

Trends in either direction, 5 plots. Investigate for cause of progressive change.

Upper control limit

Target

Lower control limit

Figure S6.7

Patterns in Control Charts

Two plots very near lower (or upper) control. Investigate for cause.

Upper control limit

Target

Lower control limit

Figure S6.7

Patterns in Control Charts

Run of 5 above (or below) central line. Investigate for cause.

Upper control limit

Target

Lower control limit

Figure S6.7

Patterns in Control Charts

Erratic behavior. Investigate.

Upper control limit

Target

Lower control limit

Figure S6.7