Post on 08-Apr-2018
8/7/2019 Ch05 Control Charts
1/29
Control Charts
Review homework
Ch 4 8, 10, 24, 26, 36,
44, 49
Lecture/discussion
Variable control charts
SPC at Maine Medical
Week 10 assignment
Homework
Ch 5 2, 8, 18, and
control chart
handout
Read Chapters 6 and 7
Process Capability
Other Variable
Control Charts
Week 9AgendaAgenda
8/7/2019 Ch05 Control Charts
2/29
Control Charts
Control ChartsControl Charts
Chapter Five
8/7/2019 Ch05 Control Charts
3/29
Control Charts
Control chartfunctionsControl chartfunctions
Control charts are decision-making tools- they provide an economic basis fordeciding whether to alter a process orleave it alone
Control charts are problem-solving tools -they provide a basis on which to
formulate improvement actions SPC exposes problems; it does not solve
them!
8/7/2019 Ch05 Control Charts
4/29
Control Charts
Control chartsControl charts
Control charts are powerful aids to
understanding the performance of a
process over time.
PROCESSPROCESS
Input Output
Whats causing variability?
8/7/2019 Ch05 Control Charts
5/29
Control Charts
Control chartsidentifyControl chartsidentify
variationvariation Chance causes - common cause
inherent to the process or random and not
controllable if only common cause present, the process
is considered stable or in control
Assignable causes - special cause
variation due to outside influences
if present, the process is out of control
8/7/2019 Ch05 Control Charts
6/29
Control Charts
Control charts help us learnControl charts help us learn
moreaboutprocessesmoreaboutprocesses Separate common and special causes of
variation
Determine whether a process is in a stateof statistical control or out-of-control
Estimate the process parameters (mean,
variation) and assess the performance ofa process or its capability
8/7/2019 Ch05 Control Charts
7/29
Control Charts
Control chartsto monitorControl chartsto monitor
processesprocesses To monitor output, we use a control chart
we check things like the mean, range,
standard deviation To monitor a process, we typically use
two control charts
mean (or some other central tendencymeasure)
variation (typically using range or standard
deviation)
8/7/2019 Ch05 Control Charts
8/29
Control Charts
Control chartcomponentsControl chartcomponents
Centerline
shows where the process average is
centered or the central tendency of the data Upper control limit (UCL) and Lower
control limit (LCL)
describes the process spread
8/7/2019 Ch05 Control Charts
9/29
Control Charts
Control chartforvariables (ChControl chartforvariables (Ch
5)5) Variables are the measurablemeasurable
characteristics of a product or service.
Measurement data is taken and arrayedon charts.
8/7/2019 Ch05 Control Charts
10/29
Control Charts
XX--barandR chartsbarandR charts
The X-bar chart - used to detect changes
in the mean between subgroups
tests central tendency or location effects The R chart - used to detect changes in
variation within subgroups
tests dispersion effects
8/7/2019 Ch05 Control Charts
11/29
Control Charts
Step 1 Definethe problemStep 1 Definethe problem
Use other quality tools to help determine
the general problem thats occurring and
the process thats suspected of causingit.
brainstorm using cause and effect diagram,
why-why, Pareto charts, etc.
8/7/2019 Ch05 Control Charts
12/29
Control Charts
Step 2Selecta quality characteristicStep 2Selecta quality characteristic
to be measuredto be measured Identify a characteristic to study - for
example, part length or any other
variable affecting performance typically choose characteristics which are
creating quality problems
possible characteristics include: length,
height, viscosity, color, temperature,
velocity, weight, volume, density, etc.
8/7/2019 Ch05 Control Charts
13/29
Control Charts
Step 3 ChooseasubgroupStep 3 Chooseasubgroup
sizeto besampledsizeto besampled Choose homogeneous subgroups
Homogeneous subgroups are produced
under the same conditions, by the samemachine, the same operator, the same
mold, at approximately the same time.
Try to maximize chance to detect
differences between subgroups, while
minimizing chance for difference with a
group.
8/7/2019 Ch05 Control Charts
14/29
Control Charts
OtherguidelinesOtherguidelines
The larger the subgroup size, the more
sensitive the chart becomes to small
variations.
This increases data collection costs.
Destructive testing may make large subgroup
sizes infeasible.
Subgroup sizes smaller than 4 arentrepresentative of the distribution averages.
Subgroups over 10 should use S chart.
8/7/2019 Ch05 Control Charts
15/29
Control Charts
Step 4 CollectthedataStep 4 Collectthedata
Run the process untouched to gather
initial data for control limits.
Generally, collect 20-25 subgroups (100total samples) before calculating the
control limits.
Each time a subgroup of sample size n istaken, an average is calculated for the
subgroup and plotted on the control
chart.
8/7/2019 Ch05 Control Charts
16/29
Control Charts
Step 5 DeterminetrialStep 5 Determinetrial
centerlinecenterline The centerline should be the population
mean, Q
Since it is unknown, we use X doublebar, or the grand average of the
subgroup averages.
m
m
i
i!! 1X
X
8/7/2019 Ch05 Control Charts
17/29
Control Charts
Step 6 Determinetrial controlStep 6 Determinetrial control
limitslimits-- X
barchartXbarchart
The normal curve displays the
distribution of the sample averages.
A control chart is a time-dependentpictorial representation of a normal curve.
Processes that are considered under
control will have 99.73% of their graphedaverages fall within six standard
deviations.
8/7/2019 Ch05 Control Charts
18/29
Control Charts
UCL LCL calculationUCL LCL calculation
deviationstandard
3XLCL
3XUCL
!
!
!
W
W
W
8/7/2019 Ch05 Control Charts
19/29
Control Charts
Determininganalternative value forDetermininganalternative value for
thestandarddeviationthestandarddeviation
m
m
i
i
!! 1R
R
RAXUCL 2!
RAXLCL2
!
8/7/2019 Ch05 Control Charts
20/29
Control Charts
Step 7 Determinetrial controlStep 7 Determinetrial control
limitslimits--
R chartR chart The range chart shows the spread or
dispersion of the individual samples
within the subgroup. If the product shows a wide spread, then
the individuals within the subgroup are not
similar to each other.
Equal averages can be deceiving.
Calculated similar to x-bar charts;
Use D3 and D4 (appendix 2)
8/7/2019 Ch05 Control Charts
21/29
Control Charts
RR--barchartexceptionsbarchartexceptions
Because range values cannot be
negative, a value of 0 is given for the
lower control limit of sample sizes of sixor less.
8/7/2019 Ch05 Control Charts
22/29
Control Charts
Step 8 Examinethe processStep 8 Examinethe process--Interpretthe charts
Interpretthe charts A process is considered to be stable
and in a state of control, or undercontrol, when the performance of the
process falls within the statistically
calculated control limits and exhibits
only chance, or common causes.
8/7/2019 Ch05 Control Charts
23/29
Control Charts
Consequences ofConsequences of
misinterpretingthe processmisinterpretingthe process Blaming people for problems that they cannot
control
Spending time and money looking for problems
that do not exist
Spending time and money on unnecessary
process adjustments
Taking action where no action is warranted
Asking for worker-related improvements when
process improvements are needed first
8/7/2019 Ch05 Control Charts
24/29
Control Charts
Process variationProcess variation
When a system is subject to only chance
causes of variation, 99.73% of the
measurements will fall within 3 standarddeviations
If 1000 subgroups are measured, 997 will
fall within the six sigma limits.
8/7/2019 Ch05 Control Charts
25/29
Control Charts
ChartzonesChartzones
Based on our knowledge of the normal curve, a
control chart exhibits a state of control when:
Two thirds of all points are near the center
value.
The points appear to float back and forth
across the centerline.
The points are balanced on both sides of thecenterline.
No points beyond the control limits.
No patterns or trends.
8/7/2019 Ch05 Control Charts
26/29
Control Charts
Identifying patternsIdentifying patterns
Trends
steady, progressive changes in level
Change, jump, or shift in level Runs - 7 points above or below; six
increasing or decreasing, clusters
Recurring cycles Two populations
Mistakes
8/7/2019 Ch05 Control Charts
27/29
Control Charts
Step 9 Revisethe chartsStep 9 Revisethe charts
In certain cases, control limits are revised
because:
out-of-control points were included in thecalculation of the control limits.
The process is in-control but the within
subgroup variation significantly improves.
8/7/2019 Ch05 Control Charts
28/29
Control Charts
Revisingthe chartsRevisingthe charts
Interpret the original charts
Isolate the causes
Take corrective action Revise the chart
Only remove points for which you can
determine an assignable cause
8/7/2019 Ch05 Control Charts
29/29
Control Charts
Step 10Achievethe purposeStep 10Achievethe purpose
Our goal is to decrease the variation
inherent in a process over time.
As we improve the process, the spread ofthe data will continue to decrease.
Quality improves!!