Title: ISQA 572/ 449 Models for Quality Control/ Process Control and Improvement
1ISQA 572/ 449Models for Quality Control/ Process
Control and Improvement
- Dr. David Raffo
- Tel 725-8508, Fax 725-5850
- Email davidr_at_sba.pdx.edu
2Agenda
- Announcements
- Questions
- HW1
- Control Charts (Variables) Cont.
- Quality Costs
- SPC Vs SQC (Inspections and Acceptance Sampling)
- Control Charts (Attributes)
- Process Capability
3Quality Costs
4Quality Costs
5Quality Cost Traditional View TM2-5
6Competitive Benefits of TQM Exhibit 2-8
7SPC Vs SQC (Inspections and Acceptance Sampling)
8Approaches to Quality TM 4-1
9Quality Control Modes TM 4-2
10Statistical Process Control Prevention
TM 4-3
11Disadvantages of Inspection
- Wasteful
- Sampling and inspection add cost and decrease
value - Inaccurate
- Even 100 inspection is only 80 effective
because of the possibility of human errors - Impractical(Costly)
- Inspection may involve destructive testing
12Disadvantages of Inspection
- Wrong message
- Inspection communicates to people and suppliers
that bad parts will still be tolerated. - Risks
- In sampling and inspection there is a risk of
accepting bad lots and rejecting good lots - No continuous improvement
- Sampling is still inspection, not prevention, so
that quality is not typically continuously
improved.
13Advantages of a Stable Process
- Management and workers know the process
capability and can predict performance, costs and
quality levels. - Productivity will be at a maximum and costs will
be minimized. - Management will be able to measure the effects of
changes in the system with greater speed and
reliability.
14Advantages of a Stable Process
- If management wants to alter specification
limits, it will have the data to back up its
decision. - (A stable process does not necessarily meet specs
nor exhibit minimal variation - its just
predictable)
15Acceptance Sampling
- Acceptance sampling has three basic decisions
accept, reject, or resample. - Reason for using acceptance sampling
- Cost of passing defects is low
- Destructive testing is required
- Cost of inspection high relative to cost of loss
- Assumes stable process
- Large number of items must be processed in a
short time
16Acceptance Sampling
- Terms
- Producers Risk (?) Risk of rejecting a lot
with acceptable quality level. (type I error) - Consumers Risk (?) Risk of accepting a lot
with unacceptable quality level. (type II error) - Acceptable Quality Level (AQL) The maximum
percentage defective that can be considered
satisfactory. - Lot Tolerance Percent Defective (LTPD) The
percent defective where the consumer desires the
probability of acceptance to be at a low level.
17Acceptance Sampling - Attributes
- Types of plans
- Single
- N, n, c
- (1000, 50, 1)
- Double
- N, n1, n2, c1, c2 , c3
- (3000, 50, 80, 1, 3, 5)
- Sequential
- n, ca, cr
- (50, 0, 4) (50, 1, 5)
18Acceptance Sampling - Attributes
- Measures
- Average Outgoing Quality (AOQ)
- Average Total Inspection
- Average Sample Number
- Standard Sampling Plans
- MIL-STD-105E
- Dodge-Romig
- Chain Sampling
- Skip-Lot
- Deming kp
19Acceptance Sampling - Variables
- Advantages
- Smaller sample than equivalent attribute plan
- Provides more information
- Provides insight into quality improvements
- Disadvantages
- Separate plan for each variable
- Inspection costs are higher
- Distribution estimate required
20Acceptance Sampling - Variables
- Process Parameter
- Average quality of the product/process or
variability of the quality is known - Single Specification
- n Xa
- Double Specification
- n, XLa, XUa
- Lot Proportion Nonconforming
- Form 1 (k-method)
- Form 2 (M-method)
21Control Charts (Attributes)
22Advantages Disadvantages of Attribute Charts
- Advantages
- Some quality characteristics can only be viewed
as a attribute. - Quality characteristic may be measurable as a
variable but an attribute is used for time, cost
or convenience. - Combination of variables can be measured as an
attribute rather than use a multivariate chart.
23Advantages Disadvantages of Attribute Charts
- Disadvantages
- Attributes dont measure the degree to which
specifications are met or not met. - Doesnt provide much information on cause.
- Variable chart can indicate potential changes
which allow preventive actions. - Larger sample size required.
24Types of Attribute Charts
- p-Chart - Fraction Nonconforming
- Can have constant or variable sample size.
- Good tool for relating information about average
quality level. - np-Chart - Number of Nonconforming
- Number of nonconforming items may be easier for
user to understand.
25Types of Attribute Charts
- c-Chart - Number of Nonconformities
- Used when desire is to control the number of
defects where one defect may not cause the entire
product to be defective. - Often used where area of opportunity is
continuous and a constant size
26Types of Attribute Charts
- u-Chart - Number of Nonconformities/unit
- Area of opportunity is of variable size.
- U-Chart - Number of Demerits/unit
- Allows the use of variable weights for different
classes of defects.
27p Chart
TM 4-12
28p Chart
TM 4-13
29p Chart
TM 4-14
30p-Chart
Exhibit 4-26
31Hotel Suite Inspection - Defects Discovered
Exhibit 4-27
32C-Chart Calculations
- Centerline c-bar (S c)/m sub-groups
- UCLc c-bar 3sqrt(c-bar)
- LCLc c-bar - 3sqrt(c-bar)
33c Chart for Hotel Suite Inspection Exhibit
4-28
34Process Capability
35Process Capability Analysis
- Creates uniformity of output
- Level of quality is maintained or improved
- Facilitates product and process design
- Assists in supplier selection and control
- Reduces total costs
36Process Capability Normal Curve TM 4-15
37Process Capability TM 4-17
38Capability Indexes
- Cp
- Ability to meet two-sided specification limits
- Cp (USL-LSL)/(6? )
- Assumes
- Stable process
- Normal distribution
- Variables data
- Centered process
- Goal Cpgt1.0
39Capability Indexes
- Capability Ratio
- CR (6?)/(USL-LSL)
- Poor if CRgt1
40Capability Indexes
- CPU CPL
- Ability to meet one-sided specification limit
- CPU (USL-X)/(3?)
- CPL (X-LSL)/(3?)
- Assumes
- Stable process
- Normal distribution
- Variables data
41Process Capability Chart Exhibit
4-20
42Capability Indexes
- Cpk
- Ability to meet two sided specification but the
process does not have to be centered - Cpk Cp - m-X/(3?) where mnominal
centerline
43Process Capability Index TM
4-18
44Process Capability Varieties
TM 4-19
45(No Transcript)