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Session XV
“Control Charts For AttributesC-Chart”
Control Charts for Count of Non-Conformities The nonconformities chart controls the count of
nonconformities within the product or service.
An item is classified as a nonconforming unit whether it has one or many nonconformities.
Count of nonconformities (c) chart.
Count of nonconformities per unit (u) chart.
Control Charts for Count of Non-Conformities Since these charts are based on the Poisson
distribution, two conditions must be met:1. The average count of nonconformities must be much
less than the total possible count of nonconformities.
2. The occurrences are independent.
Control Charts for Count of Non-ConformitiesObjectives:
1. Determine the average quality level (initial process capability).
2. Bring to the attention of management any changes in the average.
3. Improve the product quality: Ideas for quality improvement.
Control Charts for Count of Non-Conformities4. Evaluate the quality performance of operating and
management personnel.
5. Suggest places to use X-bar and R chart.
6. Determine acceptance criteria of a product before shipment to the customer.
Construction of Control Charts for Count of Non-Conformities
1. Select the quality characteristic(s):
a) Single quality characteristic.
b) Group of quality characteristics.
c) A part.
d) An entire product.
e) A number of products.
f) It can be established for performance control
of an: operator, work center, department,
shift, plant, or corporation
Construction of Control Charts for Count of Non-Conformities
2. Determine the subgroup size and method:
3. Collect the data:
At least 25 subgroups.
Different sources.
Construction of Control Charts for Count of Non-Conformities
3
3
cc
g
UCL c c
LCL c c
4. Calculate the trial central line and the control
limits:
Control Chart for Count of Nonconformities ( c Chart), Using Preliminary Data
Construction of Control Charts for Count of Non-Conformities
Construction of Control Charts for Count of Non-Conformities
d
new 0
d
0 0
0 0
c - cc = c =
g - g
UCL= c + 3 c
LCL= c - 3 c
5. Establish the revised central line and control
limits
Construction of Control Charts for Count of Non-Conformities
Construction of Control Charts for Count of Non-Conformities
6. Achieve the objectives: The reason for the
control chart is to achieve one or more of the
previously stated objectives.
Control Charts for Count of Non-Conformities/Unit (u Chart)
Chart for Count of Nonconformities/Unit (u Chart)
3
3
ccu u
n n
uUCL u
n
uLCL u
n
u Chart for Errors on Waybills
Control Charts for Count of Non-Conformities/Unit (u Chart)
Control Charts for Count of Non-Conformities/Unit (u Chart) Scale selected is continuous for the u chart. For the
c chart is discrete.
Subgroup size for the u chart can vary. For the cchart is 1.
The u chart is limited in that we do not know the location of the nonconformities.
A Quality Rating System
Nonconformity Classification:
1. Critical nonconformities: Unsafe conditions for individuals
using, maintaining, or depending upon the product.
2. Major nonconformities: Result in failure or reduce materially
the usability of the product for its intended purpose.
3. Minor nonconformities: Reduce materially the usability of
the product for its intended purpose.
Control Chart Selection
Quality Characteristic
Variable Attribute
n>1?
n>=10?
x and MRno
yes
x and s
x and Rno
yes
Defective Defect
constant
sample
size?
p-chart with
variable sample
size
no
p or
np
yes Sampling
Unit one
c u
yes no
Computer Program
• EXCEL program files on the website will solve for:• p chart
• np chart
• c chart
• U chart
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