Ch04 spc

Quality
Improvement

Statistical Process
Control

Outline
 Pareto Diagram
 Cause-Effect Diagram
 Check Sheets
 Process Flow Diagram
 Scatter Diagram
 Histogram
 Control Charts

2

Learning Objectives
When you have completed this chapter you
should be able to:
 Construct

a Pareto diagram.

 Explain

how to construct a cause and effect
diagram.

 Explain

how to construct a check sheet.

 Construct

a process flow chart.

3

Statistical Process Control
A methodology for monitoring a process
to identify special causes of variation
and signal the need to take corrective
action when appropriate

4

The Pareto Principle
Vilfredo Pareto was an economist who is credited
with establishing what is now widely known as the
Pareto Principle or 80/20 rule. When he discovered
the principle, it established that 80% of the land in
Italy was owned by 20% of the population. Later, he
discovered that the pareto principle was valid in
other parts of his life, such as gardening: 80% of his
garden peas were produced by 20% of the peapods.

5

The Pareto Principle
Some Sample 80/20 Rule Applications
 80% of process defects arise from 20% of

the process issues.

 20% of your sales force produces 80% of

your company revenues.

 80% of delays in schedule arise from 20% of

the possible causes of the delays.

 80% of customer complaints arise from 20%

of your products or services.
(The above examples are rough estimates.)

6

The Pareto Diagram
 Graph that ranks data classifications in

descending order from left to right
 Pareto diagrams are used to identify the most

important problems
 Advantage: Provide a visual impact of those

vital few characteristics that need attention
 Resources are then directed to take the

necessary corrective action
7

The Pareto Diagram
 Helps a team focus on causes that have the

greatest impact
 Displays the relative importance of problems

in a simple visual format
 Helps prevent “shifting the problem” where

the solution removes some causes but
worsens others

8

Constructing a Pareto Diagram
Steps:
1.

Determine the method of classifying the
data: by problem, cause, type of
nonconformity, etc

2.

Decide if dollars (best), weighted frequency,
or frequency is to be used to rank the
characteristics

3.

Collect data for an appropriate time interval
9

Constructing a Pareto Diagram
Steps cont’d:
4.

Summarize the data and rank order
categories from largest to smallest

5.

Compute the cumulative percentage if it is
to be used

6.

Construct the diagram and find the vital few

10

Cause-and-Effect Diagram
 It was developed by Dr. Kaoru Ishikawa in 1943
 Picture composed of lines and symbols

designed to represent a meaningful relationship
between an effect and its causes
 Effect (characteristics that need improvement)

on the right and causes on the left

12


Figure 4.3 Cause-and-Effect Diagram
13

 Enables a team to focus on the content of a

problem, not on the history of the problem or
differing personal interests of team members
 Creates a snapshot of collective knowledge and

consensus of a team; builds support for solutions
 Focuses the team on causes, not symptoms
 Used to investigate either a “bad” effect and to

take action to correct the causes or a “good” effect
and to learn those causes responsible
14

Steps in the construction of a
Cause-and-Effect Diagram:
1.

Identify the effect or quality problem

2.

Determine the major causes

3.

Determine all the minor causes. Request a
brainstorming session

4.

Once the diagram is complete, evaluate it to
determine the most likely causes

5.

Develop solutions
15

Advantages:
1.

Analyzing actual conditions for the purpose of
product or service quality improvement

2.

Elimination of conditions causing
nonconforming product or service and
customer complaints

3.

Standardization of existing and proposed
operations

4.

Education and training in decision-making
16

Types of Diagrams:
1.

The dispersion-analysis type.


Each major branch is filled in completely
before starting work on any of the other
branches. The objective is to analyze the
causes of dispersion or variability

17

Types of Diagrams:
2.

The process-analysis type.


In order to construct it, it is necessary to
write each step of the production process.
The advantage of this type of diagram is
the ease of construction and its simplicity,
since it follows the production sequence

18

Check Sheets
 The main purpose is to ensure that the data

are collected carefully and accurately by
operating personel for process control and
problem solving
 The form of the check sheet is individualized

for each situation and is designed by the
project team
 Check sheets are designed to show location

20

Check Sheets
 Creates easy-to-understand data
 Builds, with each observation, a clearer picture

of the facts
 Forces agreement on the definition of each

condition or event of interest
 Patterns in the data become obvious quickly
 Creativity plays a major role in the design of a

check sheet
21

Check Sheets

Figure 3-5 Check Sheet for paint non-conformities

27

Figure 3-6 Check Sheet for swimming pool
28

Process Flow Diagram
 It is a schematic diagram that shows the flow

of the product or service as it moves through
the various processing stations or operations

 Makes it easy to visualize the entire system,

identify potential trouble spots, and locate
activities

 Compares and contrasts actual versus ideal

flow of a process

25

 Serves as a training tool
 Uses standardized symbols
 Shows unexpected complexity, problem

areas, redundancy, unnecessary loops, and
where simplification may be possible

27

 Allows a team to reach agreement on process

steps and identify activities that may impact
performance

 Improvements to the process can be

accomplished by eliminating steps, combining
steps, or making frequently occurring steps
more efficient

28

Scatter Diagram
The simplest way to determine if a cause
and-effect relationship exists between two
variables. Details are in Chapter 5.

29

Histogram
Graphically shows the process capability and,
if desired, the relationship to the
specifications and the nominal

Figure 3-11 Histogram for Hole Location
30

Histogram
 Displays large amounts of data that are difficult

to interpret in tabular form

 Shows centering, variation, and shape
 Illustrates the underlying distribution of the data
 Provides useful information for predicting future

performance

 Helps to answer the question “Is the process

capable of meeting requirements?

31

Control Charts
 Focuses attention on detecting and

monitoring process variation over time

 Distinguishes special from common causes of

variation

 Serves as a tool for on-going control
 Provides a common language for discussion

process performance

 Details in Chapters 6,7, & 9
32

Ch04 spc

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