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