Quality Control
What is Quality Control?
Juran and Godfrey define Quality Control as “a managerial process
for conducting operations so as to provide stability – to prevent
adverse change and to “maintain the status quo”
To maintain stability, quality control process includes the following:
Evaluates actual performance
Compares actual performance to goals
Takes action on the differences
Another definition: Operational techniques and activities used to fulfill
quality requirements.
The difference between QC and QA
Quality Control (QC) and Quality Assurance (QA) compare
performance to goals. However !!!!!!
In QC performance is evaluated during operations and performance
is compared to goals during operations and the action is taken by the
operating force.
In QA performance is evaluated and compared after operations and
resulting information is provided to operating forces and others who
have needs to know.
The Feedback Loop
4- The process responds by
restoring conformance
3- The actuator stimulates
the process to change the
performance to meet the
standards
1- To evaluate the actual
quality of the control subject
2- The umpire receives
information from the sensor
and compares them to the
quality goals and standards
Think of the thermostat to control temperature and the cruise control
used in cars to control speed
A flowchart for the quality control process
The 7 Quality Control tools
1. Check Sheets
2. Scatter Diagrams
3. Cause-and-Effect Diagrams
4. Pareto Charts
5. Flowcharts
6. Histograms
7. Control Charts
1- Check Sheets
A check sheet is an organized method of recording data so that
decisions can be based on facts rather than anecdotal evidence.
Check Sheet items should be selected to be mutually exclusive and
to cover all reasonable categories.
1- Check Sheets
The following figure shows a check sheet used to determine the
causes of defects in a hypothetical assembly process.
If we had too many defects under the category “others”, a new set
of categories is needed
2- Scatter Diagrams
Scatter Diagrams show the relationship between two measurements.
One measurement is plotted on the horizontal axis (x-axis) and
the other is plotted on the vertical axis (y-axis). The distribution
of their intersection points reveals the relationship pattern.
2- Scatter Diagrams
2- Scatter Diagrams
The Correlation Coefficient r
( x  x )( y  y )

r
(n  1) S x S y
The Sample S tan dard Deviation for x
Sx 

2
xx)
n 1
The Sample S tan dard Deviation for y
Sy 

2
y y)
n 1
2- Scatter Diagrams
The sales manager of Copier Sales company wants to determine
whether there is a relationship between the number of sales calls
made in a month and the number of copiers sold that month.
The manager selects a random sample of 10 representatives and
determines the number of sales calls each representative made last
month and the number of copiers sold.
2- Scatter Diagrams
Does this mean that more sales calls cause more sales?
No, we have not demonstrated cause and effect here, only
that the two variables are related.
3- Cause-and-Effect Diagrams
Known as the Fishbone charts and the Ishikawa diagrams
Ishikawa diagrams are used to identify and understand the factors
that are causing an undesired effect. The factors are identified by
people who are familiar with the process involved.
Contributing factors (Categories) could be the “four M’s”: Method,
Manpower, Material, and Machinery or the “four P’s”: Policies,
Procedures, People, and Plant.
Individual causes associated with each categories are tied in a
separate bones along that branch, often through a brainstorming
process.
3- Cause-and-Effect Diagrams
Contributing factors are not root causes. The team needs to examine
the contributing factors to find the root causes. This can be done by
digging deeper – asking repeated “why” questions of the
contributing factors.
Example: for a basketball quality control problem- missed free
throws. Each bone represents a possible source of error (category or
factor)
The figure in the following slide showed that the Method category
has problems caused by hand positions, follow-through, aiming point,
bent knees, and balance. Further causes for the sub-branches can be
developed and analyzed.
3- Cause-and-Effect Diagrams
Method
(shooting process)
Material
(ball)
Size of ball
Grain/Feel
(grip)
Aiming point
Bend knees
Air pressure
Hand position
Lopsidedness
Follow-through
Training
Conditioning
Consistency
Missed
free-throws
Rim size
Motivation
Rim alignment
Concentration
Manpower
(shooter)
Balance
Machine
(hoop &
backboard)
Rim height
Backboard
stability
4- Pareto Charts
Pareto charts are a method of organizing errors, problems, or
defects to help focus on problem solving efforts. They are used to
identify a set of priorities.
They are based on the work of Vilfredo Pareto, a 19th-centory
economist who noted that a few people controlled most of the
nation’s wealth.
Joseph Juran popularized Pareto’s work when he suggested that
80% of a firm’s problems are a result of only 20% of the causes.
Separating the “vital few” from the “trivial many” can be done using
the Pareto Charts.
4- Pareto Charts
Using the data from the hypothetical assembly process
4- Pareto Charts
Data for October
Frequency (number)
60 –
54
– 72
50 –
40 –
Number of
occurrences
30 –
20 –
10 –
0 –
Room svc
72%
12
4
3
Check-in Pool hours Minibar
16%
5%
4%
2
Misc.
3%
Causes and percent of the total
Cumulative percent
– 100
– 93
– 88
70 –
5- Flowcharts
Flowcharts graphically present a process or system using annotated
boxes and interconnected lines.
They are a simple but great tool for trying to make sense of a
process or explain a process.
Flowcharts should reflect the actual process used rather than what
the process owner thinks it is or wants it to be.
We can get a brief idea about the critical path of the process and
the events involved in the critical path.
Measurements could be taken at each step to find the significant
causes of delays.
5- Flowcharts
A hospital is undertaken a series of process improvement initiatives.
One of these is to make the MRI service efficient for patients, doctors,
and hospital. The first step is to develop a flowchart for this process
1.
2.
3.
4.
5.
6.
Physician schedules MRI
Patient taken to MRI
Patient signs in
Patient is prepped
Technician carries out MRI
Technician inspects film
7.
8.
9.
10.
If unsatisfactory, repeat
Patient taken back to room
MRI read by radiologist
MRI report transferred to
physician
11. Patient and physician discuss
8
1
2
3
4
5
6
7
80%
11
9
20%
10
6- Histograms
Histograms show the range of values of a measurement and the
frequency with which each value occurs.
They show the most frequently occurring reading as well as the
variations on the measurements.
Descriptive statistics, such as the average and the standard
deviation, may be calculated to describe the distribution.
The visual presentation of the distribution may also provide insight
into the cause of variation.
6- Histograms
Example
Another example could be the time spent to take a blood sample. If
the intended average is in the middle and the variation is not that
high then thing could be accepted.
Assignment 2
For the same group distribution
Each group is asked to do the following
1. Find a real time problem at work and use the cause-and-effect
diagrams to find the causes of the problem. (please explain the
problem clearly in one slide)
2. Use a large set of data (greater that 500 entity) then use one of
the following tools to explain the data and find any interesting
remarks (Scatter Diagrams, Pareto Charts, or Histograms)
3. Prepare a ppt presentation to share your findings with your
classmates next week. (I expect to receive your ppt slides and the
data file at 11:30 am of the day of the lecture)