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Lean 6 Sigma > Lecture 5 - MSA & Process Capability > Flashcards

Lecture 5 - MSA & Process Capability Flashcards

1
Q

What does performance measurement of core business processes involve?

A
  1. Determine which data to capture.
  2. Develop a data collection plan for the process & collect data.
  3. Establish baseline performance.
  4. Compare to customer results to determine the shortfall.
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2
Q

Terminologies of DPMO:

What is a Unit (U)?

A
  • Parts
  • Components
  • Assemblies
  • Sub assemblies
  • Systems tested or inspected
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3
Q

Terminologies of DPMO:

What is CTQ?

A

The characteristic of a product/process that is Critical To Quality in the eyes of the customer.

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4
Q

Terminologies of DPMO:

What is a defect (D)?

A

Non-conformity in a Unit.

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5
Q

Terminologies of DPMO:

What is an Opportunity (O)?

A

The total number of chances per unit to have a defect. Anything that can be measured, or tested for the presence of a defect is opportunity. There could be multiple opportunities in a unit that could lead to a defect.

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6
Q

Calculating DPMO: How do you calculate

Total opportunities (TOP)?

A

TOP = U x O

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7
Q

Calculating DPMO: How do you calculate

Defect per unit (DPU)?

A

DPU = D / U

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8
Q

Calculating DPMO: How do you calculate

Defects per unit opportunity (DPO)?

A

DPO = DPU / O

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9
Q

Calculating DPMO: How do you calculate

Defects per million opportunity (DPMO)?

A

DPMO = DPO x 10⁶

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10
Q

What is the goal of a Six Sigma project?

A

To improve Sigma quality level and not necessarily to reach Six Sigma performance immediately.

Decide Sigma level that is good enough, then compare your process to competitors - most important is that you are better than them!

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11
Q

What does Measurement System Analysis (MSA) involve?

A
  1. Selecting the correct measurement and approach
  2. Assessing the measuring device
  3. Assessing procedures and people conducting measurement
  4. Assessing any measurement interactions (people, measurement device and object measured)
  5. Calculating the measurement uncertainty of individual measurement devices and/or measurement systems
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12
Q

MSA elements

What is accuracy in terms of a measurement system?

A

The difference between the measurement and the part’s actual value.

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13
Q

MSA elements

What is precision in terms of a measurement system?

A

The variation you see when you measure the same part repeatedly with the same gauge.

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14
Q

Accuracy & Precision related terms

What is true value?

A

Theoretically correct value

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15
Q

Accuracy & Precision related terms

What is bias?

A

Difference between the average value of all measurements of a sample and the true value for that sample.

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16
Q

Accuracy & Precision related terms

What is stability?

A

Stable processes are those that are free from special cause variation.

17
Q

Accuracy & Precision related terms

What is linearity?

A

A measure of any change in accuracy or precision over the range of instrument capability.

18
Q

Accuracy & Precision related terms

What is repeatability?

A

Variability inherent in the measurement system under constant conditions.

or

Variation when one person repeatedly measures the same unit with the same measuring equipment.

One piece - one measuring equipment - one operator

19
Q

Accuracy & Precision related terms

What is reproducibility?

A

Variability among measurement made under different conditions (e.g. different operators, measuring devices, etc.)

or

Variation between means when two or more people measure the same unit with the same measuring equipment.

One piece - one measuring equipment - various operators

20
Q

Describe accuracy and precision in terms of a target.

A

Accurate but not precise - On average, the shots are in the centre of the target but there is a lot of variability.

Precise but not accurate - The average is not on the centre, but the variability is small.

21
Q

Name some sources of poor measurement.

A
  • Poor or non-existent operational definitions
  • Lack of understanding of the definitions
  • Difficult measures
  • Poor sampling
  • Inaccurate, insufficient or non-calibrated measurement devices
22
Q

What is an operational definition of measurement?

A

A precise description of how to obtain the measurements you require.

It defines exactly what and how to measure (including how to use the measurement equipment).

23
Q

What are specification limits?

A

The allowable range of product deviation from the customer’s requirements that he/she is willing to accept.

It is derived from customer requirements.

24
Q

Specification Limits in pub

Who sets the lower specification limit (LSL)?

A
  • Government: “UK law requires certain steps be taken to ensure that a pint of beer is indeed a pint… Until recently glasses had a crown stamp and number etched upon them indicating that the certification had been done by an agency of the Crown.”
  • Customer
25
Q

Specification Limits in pub

Who sets the upper specification limit (USL)?

A
  • Glass capacity

- Profitability of pub

26
Q

What is process capability?

A
  • Ability of a process to meet its specifications
  • Measures the performance of a process
  • Process must be stable in order to calculate process capability (only common cause variation)

The two types of process capability measures are:

  • Cp Index: measures the process potential
  • Cpk Index: measures the process capability but takes into account the ‘off-centredness’ of a process
27
Q

Equation for Cp index is:

A

Cp = (USL-LSL)/6σ

= Voice of the customer / Voice of the process

or

= specification width / process width

28
Q

What does Cp < 1 imply?

A

Process incapable

Inherent or natural variability in the process, as measured by the process spread 6σ, is greater than the difference between the specification limits.

Cp < 0.67:            Terrible capability rating
0.67 < Cp < 1.00:  Poor
1.00 < Cp < 1.33:   Fair
1.33 < Cp < 1.67:   Good
1.67 < Cp < 2.00:  Excellent
Cp = 2.00:            Terrific
29
Q

Process capability yogurt example

Factory producing and packaging yogurt in 500g pots. The process mean is 500g and the standard deviation is 2g. The LSL is 495g and the USL is 510g.

What is the process capability, Cp?

A

Cp = (510 - 495) / (6 x 2) = 15/12 = 1.25: Fair capability rating

30
Q

What are the corrective approaches to increase process potential?

A

Increase the specification limits - make sure that customers are happy with the changes of specification limits!

Reduce the process spread - investing in better raw material, new equipment / processes or experienced operators.

Side note: Many companies prefer Cp values >= 1.33 as this ensures an extremely low (0.006%) non-conformance rate.

31
Q

What are the limitations of Cp?

A
  • Does not incorporate the process mean, μ, or centres process.
  • Process can be capable and still not met the customer requirements.
32
Q

Equation for Cpk index is:

A

Cpk = min { (USL-μ) / 3σ, (μ-LSL) / 3σ }

or

Cpk = proportion of natural tolerances between process centre and nearest spec.

Desirable to have Cpk > 1

33
Q

Process capability yoghurt example

Factory producing and packaging yoghurt in 500g pots. The process mean is 500g and the standard deviation is 2g. The LSL is 495g and the USL is 510g.

a) What is the process capability, Cpk?
b) An equipment provider offers a new high-tech packaging machine with specifications of standard deviation of 0.5 g. Does it solve our problem?
c) What would happen if we used the new packaging machine and set it up for a 496.5 gr mean (instead of the current 500 g)?

A

a) Cpk = min { (510-500) / 3 x 2, (500-495) / 3 x 2 }

= min { 1.67, 0.83} = 0.83

b) Cpk = min { (510-500) / 3 x 0.5, (500-495) / 3 x 0.5 }

= min { 6.67, 3.33} = 3.33

c) Cpk = min { (510-496.5) / 3 x 0.5, (496.5-495) / 3 x 0.5 }

= min { 9, 1} = 1

= (500-496.5) / 500 = 0.7% less yoghurt

34
Q

Introduction to Sampling

What is statistical inference?

A

Using sample data to draw conclusions about a population.

35
Q

Introduction to Sampling

What is parameter?

A

Numeric value that describes population (μ, σ)

36
Q

Introduction to Sampling

What is statistic?

A

Numeric value that describes sample (𝑥, s)

37
Q

What are the reasons for sampling?

A
  1. To save time, money and effort.
  2. Produces relatively accurate conclusions about the population from sample (if sample is representative)
    - Appropriate sampling method
    - Appropriate sample size
  3. Impossible to examine all population
    - Too large population (e.g. voters)
    - Sampling is destructive (e.g. testing wine bottles)
    - ££££
38
Q

What are the four sampling approaches?

A
  • Random sampling
  • Systematic sampling
  • Sub group sampling
  • Stratified sampling

Lean 6 Sigma (9 decks)

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