Control Charts are tools for tracking variation based on the principles of probability and statistics - PowerPoint PPT Presentation

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Control Charts are tools for tracking variation based on the principles of probability and statistics

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Calculate some characteristic like average, standard deviation, or range. ... Measurement charts: some characteristic we can measure (weight, time, distance) ... – PowerPoint PPT presentation

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Title: Control Charts are tools for tracking variation based on the principles of probability and statistics


1
SPC Statistical Process Control
  • Control Charts are tools for tracking variation
    based on the principles of probability and
    statistics

2
Variation
  • Exists in any process
  • error rate made by receptionist entering guest
    record data,
  • bus time between two points,
  • ounces of beverage in a bottle,
  • number of minutes past the alarm that you stay in
    bed in the morning.

3
Sources of Variation 2 Types
  • Common (random) causes chance or generally
    unidentifiable sources of variation.
  • Slight variation in walking speed
  • Slight variation in raw material
  • Controllable or Assignable causes A reason why
    the change occurred
  • people blunder, faulty setup, or a batch of
    defective raw material,worn equipment,
    fluctuating temperature
  • Super huge peanuts arrived from supplier so we
    only got 8 in each bag.

4
Controllable Causes
Average
Grams
(a) Location
5
Controllable Causes
Average
Grams
(b) Spread
6
Controllable Causes
Average
Grams
(c) Shape
7
The Normal Distribution
? Standard deviation
8
How variation impacts our process
  • Generally random variation cannot economically be
    eliminated from a process.
  • Controllable variation can be detected and
    elimination of its causes is economically
    justified.
  • Observations beyond the control limits are
    attributed to controllable variation.

9
Process Control Chart Activities
  • Periodically sample from our operation or
    process.
  • Calculate some characteristic like average,
    standard deviation, or range.
  • Plot the characteristic in time order on the
    chart.

10
Purpose of Charts
  • To ensure the process variation is in control
  • To ensure that the process is capable of meeting
    the requirements (specifications and tolerances
    of the organization)

11
Variable Control Charts (X bar R)
  • Measurement charts some characteristic we can
    measure (weight, time, distance)
  • X average measurement for the sample
  • R range of the measurements in the sample
  • Variable charts have lots of information, better
    for advanced analysis of a process

12
Control Limit Formulas Constants (A2, D3, D4)
In Textbook pg. 167
Our sample size n ?
13
Control Chart Examples
UCL
Nominal
Variations
LCL
Sample number
Appears to have normal variation
14
Control Chart Examples
UCL
Nominal
Variations
LCL
Sample number
A process with a gradual trend
15
Control Chart Examples
UCL
Nominal
Variations
LCL
Sample number
Points Outside of the Control limit
16
Attribute Chart P-Charts
  • Number of defective units in a sample. Yes/No,
    Pass/Fail, Go/No Go criteria
  • p- easy to measure (pass/fail) but sample size
    must be big enough to detect at least one
    defective item on average
  • ppercentage faulty in sample
  • N size of the sample

17
Control Charts for Attributes
P-Chart
18
Six Sigma Quality
When a process operates with ?6s variation inside
the tolerance limits, only 2 parts out of a
million will be unacceptable.
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