Statistical process control ppt @ bec doms
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Transcript of Statistical process control ppt @ bec doms
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Statistical Process Control
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Outline
STATISTICAL PROCESS CONTROL (SPC) Control Charts for Variables The Central Limit Theorem Setting Mean Chart Limits Setting Range Chart Limits (R-Charts) Using Mean and Range Charts Control Charts for Attributes Managerial Issues and Control Charts
Charts)-x(
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Outline
PROCESS CAPABILITY Process Capability Ratio (Cp)
Process Capability Index (Cpk
ACCEPTANCE SAMPLING Operating Characteristic (OC) Curves Average Outgoing Quality
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When you complete this chapter, you should be able to Identify or Define: Natural and assignable causes of variation Central limit theorem Attribute and variable inspection Process control LCL and UCL p-charts and C-charts
Learning Objectives
charts-Randcharts-x
pkp CandC ˆ
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When you complete this chapter, you should be able to Identify or Define: Acceptance sampling OC curve AQL and LTPD AOQ Producer’s and consumer’s risk
Learning Objectives - Continued
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Learning Objectives - Continued
When you complete this chapter, you should be able to Describe or explain: The role of statistical quality control
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Measures performance of a process Uses mathematics (i.e., statistics) Involves collecting, organizing, & interpreting data Objective: provide statistical signal when
assignable causes of variation are present Used to
Control the process as products are produced Inspect samples of finished products
Statistical Quality Control (SPC)
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StatisticalQuality Control
Process Control
Acceptance Sampling
Variables Charts
Attributes Charts
Types of Statistical Quality Control
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Natural and Assignable Variation
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Characteristics for which you focus on defects
Classify products as either ‘good’ or ‘bad’, or count # defects e.g., radio works or not
Categorical or discrete random variables
AttributesVariables
Quality Characteristics
Characteristics that you measure, e.g., weight, length
May be in whole or in fractional numbers
Continuous random variables
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Statistical technique used to ensure process is making product to standard
All process are subject to variability Natural causes: Random variations Assignable causes: Correctable problems
Machine wear, unskilled workers, poor material
Objective: Identify assignable causes Uses process control charts
Statistical Process Control (SPC)
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Process Control: Three Types of Process Outputs
Frequency
Lower control limit
Size(Weight, length, speed,
etc. )
Upper control limit
(b) In statistical control, but not capable of producing within control limits. A process in control (only natural causes of variation are present) but not capable of producing within the specified control limits; and
(c) Out of control. A process out of control having assignable causes of variation.
(a) In statistical control and capable of producing within control limits. A process with only natural causes of variation and capable of producing within the specified control limits.
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The Relationship Between Population and Sampling
Distributions
Uniform
Normal
BetaDistribution of sample means
x means sample of Mean
n
xx
Standard deviation of
the sample means
(mean)
x2 withinfall x all of 95.5%
x3 withinfall x all of 99.7%
x3 x2 x x x1 x2 x3
Three population distributions
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Sampling Distribution of Means, and Process Distribution
Sampling distribution of the means
Process distribution of the sample
)mean(
mx
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Process Control Charts
Plot of Sample Data Over Time
0
20
40
60
80
1 5 9 13 17 21
Time
Sam
ple
Val
ue
SampleValueUCL
Average
LCL
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Show changes in data pattern e.g., trends
Make corrections before process is out of control
Show causes of changes in data Assignable causes
Data outside control limits or trend in data
Natural causes Random variations around average
Control Chart Purposes
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X
As sample size gets large enough,
sampling distribution becomes almost normal regardless of population distribution.
Central Limit Theorem
XX
Theoretical Basis of Control Charts
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X
Mean
Central Limit Theorem
x
x
n
xx
nX X
Standard deviation
X X
Theoretical Basis of Control Charts
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ControlCharts
RChart
VariablesCharts
AttributesCharts
XChart
PChart
CChart
Continuous Numerical Data
Categorical or Discrete Numerical Data
Control Chart Types
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Statistical Process Control Steps
Produce GoodProvide Service
Stop Process
Yes
No
Take Sample
Inspect Sample
Find Out WhyCreateControl Chart
Start
Can we assign
causes?
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Type of variables control chart Interval or ratio scaled numerical data
Shows sample means over time Monitors process average Example: Weigh samples of coffee & compute
means of samples; Plot
X Chart
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Control Chart for Samples of 9 Boxes
Variation due to natural causes
17=UCL
16=Mean
15=LCL
Variation due to assignable causes
Variation due to assignable causes
Out of control
1 2 3 4 5 6 7 8 9 10 11 12Sample Number
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X Chart Control Limits
Range for sample i
# Samples
Mean for sample i
From Table S6.1
RAxxLCL
RAxxUCL
n
R R
i
n
1i
n
xi
n
ix
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Factors for Computing Control Chart Limits
Sample Size, n
Mean Factor, A2
Upper Range, D4
Lower Range, D3
2 1.880 3.268 0
3 1.023 2.574 0
4 0.729 2.282 0
5 0.577 2.115 0
6 0.483 2.004 0
7 0.419 1.924 0.076
8 0.373 1.864 0.136
9 0.337 1.816 0.184
10 0.308 1.777 0.223
12 0.266 1.716 0.284 0.184
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Type of variables control chart Interval or ratio scaled numerical data
Shows sample ranges over time Difference between smallest & largest values
in inspection sample
Monitors variability in process Example: Weigh samples of coffee & compute
ranges of samples; Plot
R Chart
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R Chart Control Limits
Range for Sample i
# Samples
From Table S6.1
n
R R
R D LCL
R D UCL
i
n
1i
3R
4R
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Steps to Follow When Using Control Charts
1. Collect 20 to 25 samples of n=4 or n=5 from a stable process and compute the mean.
2. Compute the overall means, set approximate control limits,and calculate the preliminary upper and lower control limits.If the process is not currently stable, use the desired mean instead of the overall mean to calculate limits.
3. Graph the sample means and ranges on their respective control charts and determine whether they fall outside the acceptable limits.
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Steps to Follow When Using Control Charts - continued
4. Investigate points or patterns that indicate the process is out of control. Assign causes for the variations.
5. Collect additional samples and revalidate the control limits.
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Mean and Range Charts Complement Each Other
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Type of attributes control chart Nominally scaled categorical data
e.g., good-bad
Shows % of nonconforming items Example: Count # defective chairs & divide by
total chairs inspected; Plot Chair is either defective or not defective
p Chart
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p Chart Control Limits
# Defective Items in Sample i
Size of sample i
z = 2 for 95.5% limits; z = 3 for 99.7% limits
sample each of size where
n
xp and
k
nn
n)p(1p
zpLCL
n)p(1p
zpUCL
i
k
1i
i
k
1ii
k
1i
p
p
nn
ppp
)1(
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P-Chart for Data Entry Example
0.000.010.020.030.040.050.060.070.080.090.100.110.12
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Sample Number
Perc
ent D
efec
tive
UCLp=0.10
LCLp=0.00
0.04p
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Type of attributes control chart Discrete quantitative data
Shows number of nonconformities (defects) in a unit Unit may be chair, steel sheet, car etc. Size of unit must be constant
Example: Count # defects (scratches, chips etc.) in each chair of a sample of 100 chairs; Plot
c Chart
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c Chart Control Limits
# Defects in Unit i
# Units Sampled
Use 3 for 99.7% limits
k
c c
i
k
1i
ccLCL
ccUCL
c
c
3
3
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Patterns to Look for in Control Charts
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Deciding Which Control Chart to Use
Using an X and R chart: Observations are variables Collect 20-25 samples of n=4, or n=5, or more each from
a stable process and compute the mean for the X chart and range for the R chart.
Track samples of n observations each. Using the P-Chart:
We deal with fraction, proportion, or percent defectives Observations are attributes that can be categorized in two
states Have several samples, each with many observations Assume a binomial distribution unless the number of
samples is very large – then assume a normal distribution.
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Deciding Which Control Chart to Use
Using a C-Chart: Observations are attributes whose defects
per unit of output can be counted The number counted is often a small part of
the possible occurrences Assume a Poisson distribution Defects such as: number of blemishes on a
desk, number of typos in a page of text, flaws in a bolt of cloth
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Process Capability Ratio, Cp
process the of deviation standard
6σionSpecificat LowerionSpecificat Upper
pC
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Process Capability Cpk
population process the of deviation standard mean process x where
LimitionSpecificatLower x
or , x Limit ionSpecificatUpper
of minimum
3
3pkC
Assumes that the process is:• under control• normally distributed
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Meanings of Cpk Measures
Cpk = negative number
Cpk = zero
Cpk = between 0 and 1
Cpk = 1
Cpk > 1
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Form of quality testing used for incoming materials or finished goods e.g., purchased material & components
Procedure Take one or more samples at random from a
lot (shipment) of items Inspect each of the items in the sample Decide whether to reject the whole lot based
on the inspection results
What Is Acceptance Sampling?
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Set of procedures for inspecting incoming materials or finished goods
Identifies Type of sample Sample size (n) Criteria (c) used to reject or accept a lot
Producer (supplier) & consumer (buyer) must negotiate
What Is an Acceptance Plan?
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Shows how well a sampling plan discriminates between good & bad lots (shipments)
Shows the relationship between the probability of accepting a lot & its quality
Operating Characteristics Curve
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45 % Defective in Lot
P(Accept Whole Shipment)
100%
0%
Cut-Off1 2 3 4 5 6 7 8 9 100
Return whole shipment
Keep whole shipment
OC Curve100% Inspection
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OC Curve with Less than 100% Sampling
P(Accept Whole Shipment)
100%
0%
% Defective in LotCut-Off
1 2 3 4 5 6 7 8 9 100
Return whole shipment
Keep whole shipment
Probability is not 100%: Risk of keeping bad shipment or returning good one.
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Acceptable quality level (AQL) Quality level of a good lot Producer (supplier) does not want lots with
fewer defects than AQL rejected
Lot tolerance percent defective (LTPD) Quality level of a bad lot Consumer (buyer) does not want lots with
more defects than LTPD accepted
AQL & LTPD
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Producer's risk () Probability of rejecting a good lot Probability of rejecting a lot when fraction
defective is AQL
Consumer's risk (ß) Probability of accepting a bad lot Probability of accepting a lot when fraction
defective is LTPD
Producer’s & Consumer’s Risk
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An Operating Characteristic (OC) Curve Showing Risks
= 0.05 producer’s risk for AQL
= 0.10
Consumer’s risk for LTPD
Probability of Acceptance
Percent Defective
Bad lotsIndifference zoneGood lots
LTPDAQL
0 1 2 3 4 5 6 7 8
10095
75
50
25
10
0
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OC Curves for Different Sampling Plans
1 2 3 4 5 6 7 8 9 100
% Defective in Lot
P(Accept Whole Shipment)
100%
0%
LTPDAQL
n = 50, c = 1
n = 100, c = 2
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Average Outgoing Quality
N
)nN)(P)(P(AOQ ad
Where: Pd = true percent defective of the lot
Pa = probability of accepting the lot
N = number of items in the lot
n = number of items in the sample
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Negotiate between producer (supplier) and consumer (buyer)
Both parties attempt to minimize risk Affects sample size & cut-off criterion
Methods MIL-STD-105D Tables Dodge-Romig Tables Statistical Formulas
Developing a Sample Plan
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Statistical Process Control - Identify and Reduce Process
VariabilityLower
specification limit
Upper specification
limit
(a) Acceptance sampling –[ Some bad units accepted; the “lot” is good or bad]
(b) Statistical process control – [Keep the process in “control”]
(c) cpk >1 – [Design a process that is in control]