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1、2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialIntroduction toStatistical Process Control (SPC)2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialY axis = Weight (lbs)Y axis = Weight (lbs)10.5109.5Process = HoseFour Important
2、Properties:Four Important Properties:(1) Centering(2) Spread(3) Shape(4) Stability Over Time2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential5 ft4 ft3 ft2 ft1 ft0 ft2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential5 ft4 ft3 ft
3、2 ft1 ft0 ft2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential5 ft4 ft3 ft2 ft1 ft0 ftTimeTimeWhat has changed?Centering? _Spread? _4.03.02.0TimeTime2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential5 ft4 ft3 ft2 ft1 ft0 ftWhat
4、has changed?Centering? _Spread? _TimeTime2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential5 ft4 ft3 ft2 ft1 ft0 ftWhat has changed?Centering? _Spread? _TimeTime2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialAvgSpreadAvgSprea
5、dAvgSpreadAvgSpreadStable?Stable?Yes/NoYes/No2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialDetective-Work for a Stable ProcessDetective-Work for a Stable Process- Tracking the Y and Xs Simultaneously: Relationships- Active vs Passive Detective-Work- The Multi-
6、Vari Study: Mechanics ExampleDetective-Work for an Unstable ProcessDetective-Work for an Unstable Process- Tracking the Y: Behavior Change = Learning Opportunity - Communicate SPC Effectively: Intuitive-Visual Approach- Get the Right “Meaning” From Your Data (Signal VS Noise)- Bubba: On the Straight
7、 and Narrow Path?- Inventory Management- The GoalDMAIC = Doctors Detective-workDMAIC = Doctors Detective-work- Hotel Detective-Work - Hotel Detective-Work (Tempting to jump to conclusions/solutions?)(Tempting to jump to conclusions/solutions?)Strategy #1Strategy #1Strategy #2Strategy #2What Data To
8、Track Performance (Ys)? What Data To Track Performance (Ys)? - Hospital Transactions - Hospital Transactions (Where to shove the “Thermometers” to get data?)(Where to shove the “Thermometers” to get data?) NextNextNextNextAnalyze Phase = Diagnosis: Y= f(X,X,X,X,.,X)Analyze Phase = Diagnosis: Y= f(X,
9、X,X,X,.,X)2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential Bubba is known as “The straightest driver in Crick County”.One evening at Charlies Bar, he boasted, ”I drive so straight, even a quart of Moonshine wont throw me off target”Fed-up, Junior told him to “pu
10、t his money where his mouth is” - a $100 bet.They selected Charlie, the bar owner, to be the impartial gum dropper.The Scene:The Scene:2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialCharlie: The Impartial MediatorCharlie: The Impartial MediatorWarm, sticky, War
11、m, sticky, pre-chewed gumpre-chewed gumPrPre e-Che-Chew wed ed ChaCharli rliesesBar and Grill2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialYou make the call:Did Bubba stay straightorloose $100?Junior says, “My gosh, it looks like you were swerving all over the
12、 place right from the beginning!”“ “Wheres my $100?Wheres my $100?” ”2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialCONSIDER THIS .CONSIDER THIS .Why do the pieces of gum notWhy do the pieces of gum notland at the land at the exactexact same location? same loca
13、tion?Warm, sticky, Warm, sticky, pre-chewed gumpre-chewed gumPrPre e-Che-Chew wed ed ChaCharli rliesesBar and Grill. even when Charlie is trying his best!2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential- - Natural “Aim-to-Aim Variation”Natural “Aim-to-Aim Variat
14、ion” (due to Charlie) (due to Charlie)Sources of VariationSources of VariationTo develop common terminology, To develop common terminology, for now, lets simply call this for now, lets simply call this “Aim-to-Aim Variation”“Aim-to-Aim Variation”2001, General Electric Company, USA. All rights reserv
15、edGE ConfidentialGE ConfidentialNow what do you think:Did Bubba stay straightorloose $100?Wouldnt it be helpful to have some way of separating the steering Wouldnt it be helpful to have some way of separating the steering variation from the natural, inherent “Aim-to-Aim” Variation?variation from the
16、 natural, inherent “Aim-to-Aim” Variation?2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialSources of Variation:Sources of Variation:- Natural Aim-to-Aim Variation- Natural Aim-to-Aim VariationWe can strategically use this natural, unavoidable variation (i.e., th
17、e spread width) as a tool for detecting shifts in the trucks centering.The Control Chart ApproachThe Control Chart Approach2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialFirst, select the spread that we will First, select the spread that we will declare as the
18、“Baseline Behavior”. declare as the “Baseline Behavior”. Here, we will select Here, we will select “Natural Aim Variation”.“Natural Aim Variation”.Then, determine boundaries or limits that should Then, determine boundaries or limits that should contain virtually all (say 99.7%) of the “wiggling” con
19、tain virtually all (say 99.7%) of the “wiggling” (that is, if Charlies Aim continues to be the (that is, if Charlies Aim continues to be the only Source of Variation present).only Source of Variation present).Suppose that whenever any point lands Suppose that whenever any point lands outside these “
20、control limits”, an alarm outside these “control limits”, an alarm will sound.will sound.Like using a “Squelch” to separate Signals from the Noise:Like using a “Squelch” to separate Signals from the Noise:(Setting the squelch threshold level)(Detecting a signal being thats above and beyond the squel
21、ch threshold)2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential- - Aim Variation (only)Aim Variation (only)Sources of VariationSources of Variation- - Aim VariationAim Variation- Variation Due to Steering- Variation Due to SteeringSources of VariationSources of Va
22、riationSo, if any shifts in the centering occur, So, if any shifts in the centering occur, we will know!we will know!2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialThats how control limits separate signals from noiseUCLUCLLCLLCLNo evidence of any changes in the
23、 true “centering” of the truck.No evidence of any changes in the true “centering” of the truck.All the “wiggling” of the data points was simply due to noise.All the “wiggling” of the data points was simply due to noise.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confide
24、ntialDetective-Work for a Stable ProcessDetective-Work for a Stable Process- Tracking the Y and Xs Simultaneously: Relationships- Active vs Passive Detective-Work- The Multi-Vari Study: Mechanics ExampleDetective-Work for an Unstable ProcessDetective-Work for an Unstable Process- Tracking the Y: Beh
25、avior Change = Learning Opportunity - Communicate SPC Effectively: Intuitive-Visual Approach- Get the Right “Meaning” From Your Data (Signal VS Noise)- Bubba: On the Straight and Narrow Path?- Inventory Management- The GoalDMAIC = Doctors Detective-workDMAIC = Doctors Detective-work- Hotel Detective
26、-Work - Hotel Detective-Work (Tempting to jump to conclusions/solutions?)(Tempting to jump to conclusions/solutions?)Strategy #1Strategy #1Strategy #2Strategy #2What Data To Track Performance (Ys)? What Data To Track Performance (Ys)? - Hospital Transactions - Hospital Transactions (Where to shove t
27、he “Thermometers” to get data?)(Where to shove the “Thermometers” to get data?) NextNextNextNextAnalyze Phase = Diagnosis: Y= f(X,X,X,X,.,X)Analyze Phase = Diagnosis: Y= f(X,X,X,X,.,X)2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential2030In-Process Inventory (X)10
28、J F M A 19941995Award givenApril 1994April 1994: : Manager pleased to see the in-process inventory drop to 15.Manager pleased to see the in-process inventory drop to 15.Manager presents an award to Dept. 50 in honor of this Manager presents an award to Dept. 50 in honor of this achievement.achieveme
29、nt.Ceremony in the cafeteria: pizza and refreshments for all!Ceremony in the cafeteria: pizza and refreshments for all!“Everyone should be proud of what youve accomplished”.“Everyone should be proud of what youve accomplished”.Derived from Understanding Variation: The Key To Managing Chaos, Donald J
30、. Wheeler, SPC Press. 1993.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential2030In-Process Inventory (X)10J F M A M J J 19941995Manager regrets giving awardJuly 1994July 1994: Three consecutive months of inventory increases.: Three consecutive months of inventory
31、 increases.Manager wishes he could take back the award.Manager wishes he could take back the award.“Recognition seems to have been backfired.”“Recognition seems to have been backfired.”Instead of holding the gains, Dept.50 seems to have gotten Instead of holding the gains, Dept.50 seems to have gott
32、en complacent - allowing inventory to creep back to where it was complacent - allowing inventory to creep back to where it was previously.previously.Manager decides: “This Mr. Nice Guy stuff doesnt work”Manager decides: “This Mr. Nice Guy stuff doesnt work”Derived from Understanding Variation: The K
33、ey To Managing Chaos, Donald J. Wheeler, SPC Press. 1993.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential2030In-Process Inventory (X)10J F M A M J J A S O N D19941995No more Mr. Nice GuyNov. 1994Nov. 1994: In-process inventory rises to a value of 26!: In-process
34、 inventory rises to a value of 26!Manager decides to kick aManager decides to kick anatomy and take names. and take names.He calls everyone in and demands they do something to keep inventories down.He calls everyone in and demands they do something to keep inventories down.Following this tantrum, ev
35、eryone in Dept.50 treads lightly. Short of hiding Following this tantrum, everyone in Dept.50 treads lightly. Short of hiding material in the dark corners of the plant, they dont know what to do.material in the dark corners of the plant, they dont know what to do.So, they hold their breath and hope
36、inventories drop.So, they hold their breath and hope inventories drop.Derived from Understanding Variation: The Key To Managing Chaos, Donald J. Wheeler, SPC Press. 1993.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialJune 1995June 1995: : Manager has seen reduc
37、ed levels of inventory since the end of last Manager has seen reduced levels of inventory since the end of last year. “Things are looking-up!” year. “Things are looking-up!” (Although nothing had been done to change the system)His learning: “Tough management style gets results!”His learning: “Tough
38、management style gets results!”Manager concludes: “Tough management style gets results!”2030In-Process Inventory (X)10J F M A M J J A S O N D19941995J F M A M JDerived from Understanding Variation: The Key To Managing Chaos, Donald J. Wheeler, SPC Press. 1993.2001, General Electric Company, USA. All
39、 rights reservedGE ConfidentialGE Confidential2030In-Process Inventory (X)10J F M A M J J A S O N D19941995UCLJ F M M J J A S OLCL“ “But I always made sure my decisions/actions were based on data!But I always made sure my decisions/actions were based on data!What could be wrong with that?”What could
40、 be wrong with that?”His decisions stemmed from interpreting the high and low points as signals.However, as the control chart below clearly reveals, NONE OF THESE INDIVIDUAL POINTS WERE SIGNALS!They all reflect the same underlying system - each should be interpreted alike.Derived from Understanding
41、Variation: The Key To Managing Chaos, Donald J. Wheeler, SPC Press. 1993.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialManager concludes, “Tough management gets results!”J F M M J J A S O2030In-Process Inventory (X)10J F M A M J J A S O N D19941995Award givenM
42、anager regretsgiving awardNo more Mr. Nice GuyUCLLCLNo matter how logical and persuasive the interpretation, and no No matter how logical and persuasive the interpretation, and no matter how “right” youve been in the past - noise is still noise.matter how “right” youve been in the past - noise is st
43、ill noise.Trying to interpret noise is nothing more than an exercise in wishful Trying to interpret noise is nothing more than an exercise in wishful thinking.thinking.The result - pure fiction with no connection to reality.The result - pure fiction with no connection to reality.Derived from Underst
44、anding Variation: The Key To Managing Chaos, Donald J. Wheeler, SPC Press. 1993.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential“ “Failure to use control charts to analyze Failure to use control charts to analyze data is one of the best ways known to data is one
45、 of the best ways known to mankind to increase costs, waste effort, mankind to increase costs, waste effort, and lower morale.”and lower morale.”- Dr. Donald J. Wheeler- Dr. Donald J. Wheeler“ “Its not what we dont know that Its not what we dont know that hurts.hurts.its what we know that aint so.”i
46、ts what we know that aint so.”- Will Rogers- Will RogersFalse lessons cost us - better to know how to use data wisely!False lessons cost us - better to know how to use data wisely!Derived from Understanding Variation: The Key To Managing Chaos, Donald J. Wheeler, SPC Press. 1993.Manager concludes, “
47、Tough management gets results!”J F M M J J A S O2030In-Process Inventory (X)10J F M A M J J A S O N D19941995Award givenManager regretsgiving awardNo more Mr. Nice GuyUCLLCL2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialVideo SegmentVideo Segment( (Optional)Opt
48、ional)Watch how one managers decisions and actions are Watch how one managers decisions and actions are affected when he incorrectly interprets .affected when he incorrectly interprets .NOISENOISE (typical, inherent, minute-to-minute process variation behavior)(typical, inherent, minute-to-minute pr
49、ocess variation behavior)SIGNALSIGNAL(clearly different behavior - above and beyond the usual noise)(clearly different behavior - above and beyond the usual noise)as if it were aas if it were aDo you know any operators, co-workers, or managers who Do you know any operators, co-workers, or managers w
50、ho have managed processes this way?have managed processes this way?2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential“ “Teen Smoking Turns Upward”Teen Smoking Turns Upward”- USA Today, June,21,1994- USA Today, June,21,1994Percent By Year17.017.518.018.519.019.5199
51、21993Year1992199217.3%17.3%1993199319.0%19.0%2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialYearYear%Smokers%Smokers1984198418.818.81985198519.619.61986198618.718.71987198718.618.61988198818.118.11989198918.918.91990199019.219.21991199118.218.21992199217.317.31
52、993199319.019.093929190898887868584212019181716YearIndividualsX=18.64UCL=20.71LCL=16.573210Moving RangeR=0.7778UCL=2.541LCL=NoneI and MR Chart for %SmokersA Signal Above the Noise?A Signal Above the Noise?2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialJust this
53、.Just this.There is no evidence that the percentage of teenagers who smoke has increased. There is no evidence that the percentage of teenagers who smoke has increased. Neither is there any evidence that this percentage has decreased during the past 10 years. Neither is there any evidence that this
54、percentage has decreased during the past 10 years. The only headline for these data that has any integrity is No change in teen use of tobacco. The only headline for these data that has any integrity is No change in teen use of tobacco. Anything else is just propaganda.Anything else is just propagan
55、da.So, what can we say about the percentage of teenagers who smoke daily?So, what can we say about the percentage of teenagers who smoke daily?So how do you avoid being persuaded by propaganda?So how do you avoid being persuaded by propaganda?By the way, did you read the article about how the Trade
56、Deficit soared last April? Oh well, By the way, did you read the article about how the Trade Deficit soared last April? Oh well, thats another story - or is it?thats another story - or is it?The beginning is to realize that while all data contain noise, only some data contain signals.The beginning i
57、s to realize that while all data contain noise, only some data contain signals.If you do not know how to separate the probable noise from the potential signals, you are If you do not know how to separate the probable noise from the potential signals, you are susceptible to being misled by the noise
58、in the data. susceptible to being misled by the noise in the data. Others may use data to mislead you - or you may even mislead yourself.Others may use data to mislead you - or you may even mislead yourself. Shewharts Shewharts charts charts are the simplest way to separate signals from noise.are th
59、e simplest way to separate signals from noise.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialSelit Corp.s 4th Quarter Sales Report . . .Selit Corp.s 4th Quarter Sales Report . . . TimeTime NEastNEast SWestSWestNWestNWestNCentralNCentralMAtlanticMAtlanticSCentra
60、lSCentral1995_Q41995_Q4 952 952119611968788786706708028024624621996_Q31996_Q3 976 976119711976886887437438078074474471996_Q41996_Q4 1148 114813371337806806702702781781359359(Values In Columns = Sales In Thousands)Last QuarterLast QuarterCurrent QuarterCurrent QuarterSame QuarterSame QuarterLast Year
61、Last YearA Case Study.A Case Study.Open minitab worksheet file “Sales (Quarterly Data).mtw”Open minitab worksheet file “Sales (Quarterly Data).mtw”2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential“Kim, Mid-Atlantic sales were down 3.2% in the fourth quarter and d
62、own 2.6% from the previous year - a downward trenddownward trend! Im very disappointed in your performance. You were once my best sales representative. I had high expectations for you. Now, I can only hope that our first quarter results show some sign of life.” Kim felt her face get red. She knew sh
63、e sold more units in 1996 than in 1995. “What does Halger know anyway,” she thought to herself. “Hes just an empty suit.”802802807807781781Selit Corp.s 4th Quarter Sales Report . . .Selit Corp.s 4th Quarter Sales Report . . .Last QuarterLast QuarterCurrent QuarterCurrent QuarterSame QuarterSame Quar
64、terLast YearLast YearMid-AtlanticMid-Atlantic2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialHagler continued: “Terry, Southwest sales were super. You showed an 11.7% increase in the fourth quarter and an 11.8% increase over the previous year” - An upward trend!
65、Terry smiled. She wasnt sure how she did so well, but she sure wasnt going to change anything.119611961197119713371337Selit Corp.s 4th Quarter Sales Report . . .Selit Corp.s 4th Quarter Sales Report . . .Last QuarterLast QuarterCurrent QuarterCurrent QuarterSame QuarterSame QuarterLast YearLast Year
66、SouthWestSouthWest2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialNoise = “Common Cause”Noise = “Common Cause” Variation VariationSignal = “Assignable Cause”Signal = “Assignable Cause” Variation Variation (or “Special Cause”) (or “Special Cause”)BrainstormingBra
67、instorming: : List possible causes of List possible causes of both common and both common and assignable causesassignable causesThe strategies for improving these two types of variation are very The strategies for improving these two types of variation are very differentdifferent ! !More about this
68、later .More about this later .Process: Driving to workProcess: Driving to workMeasurement: Y = Travel timeMeasurement: Y = Travel timeExample:Example:2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialNoise = “Common Cause”Noise = “Common Cause” Variation Variation
69、Signal = “Assignable Cause”Signal = “Assignable Cause” Variation Variation (or “Special Cause”) (or “Special Cause”)BrainstormingBrainstorming: : List possible causes of List possible causes of both common and both common and assignable causesassignable causesThe strategies for improving these two t
70、ypes of variation are very The strategies for improving these two types of variation are very differentdifferent ! !More about this later .More about this later .Process: Monthly Telephone BillProcess: Monthly Telephone BillMeasurement: Y = $Measurement: Y = $Example:Example:2001, General Electric C
71、ompany, USA. All rights reservedGE ConfidentialGE Confidential203010203010“ “Special Cause” Special Cause” HighwayHighway(Signal)(Signal)“ “Common Cause” Common Cause” HighwayHighway(Noise)(Noise)Although we dont know exactly where the next drop will land, we are reasonably confident it will fall so
72、mewhere on the “Common Cause” Highway. For this reason, a process that has only common cause variation is said to be. STABLESTABLE and PREDICTABLEPREDICTABLE.A process with Special Causes might better be represented by a highway through an earthquake zone: likely to suddenly shift to the side by sev
73、eral feet or more. For this reason, a process that has Special Causes of variation is said to be. UNSTABLEUNSTABLE and UNPREDICTABLEUNPREDICTABLE.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialComponents of a Control ChartComponents of a Control ChartLower Cont
74、rol LimitLower Control LimitCentral LineCentral LineUpper Control LimitUpper Control LimitUCLLCLY-axisY-axis(Response)(Response)TIMETIMETIMETIME2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential Tracking process behavior by way of sample measurements (plotted poin
75、ts) Tracking process behavior by way of sample measurements (plotted points) can be thought of as being much like can be thought of as being much like tracking a rabbit by way of its droppings. tracking a rabbit by way of its droppings. Suppose the football-shaped droppings fall at regular 1 minute
76、intervals, and when Suppose the football-shaped droppings fall at regular 1 minute intervals, and when they hit the ground, they tend to roll around a bit (like random/sampling variation).they hit the ground, they tend to roll around a bit (like random/sampling variation).So the game becomes, How to
77、 use statistical tools as reliable indicators - to keep So the game becomes, How to use statistical tools as reliable indicators - to keep us from being fooled by the wobbling (noise) that will always be present in the us from being fooled by the wobbling (noise) that will always be present in the d
78、roppings (data) - so we reliably detect whether the rabbit did or did not move to the droppings (data) - so we reliably detect whether the rabbit did or did not move to the left or right of the straight path.left or right of the straight path.When communicating statistical concepts, you could descri
79、be SPC as follows:When communicating statistical concepts, you could describe SPC as follows:In case you might dare to use the idea one day.In case you might dare to use the idea one day.Here are some rules/principles to help us know when to react . . .Here are some rules/principles to help us know
80、when to react . . .2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential(1) Point beyond UCL or LCL (+/- 3 Std Devs)(2) 7 consecutive points on the same side of the centerline (Minitab uses 9)(3) Trend: 6 consecutive points steadily increasing or decreasing(4) Repeat
81、ed patterns/cyclesHow to Detect a Change in Process Behavior:How to Detect a Change in Process Behavior:(i.e., When an X just made an impact of the process)(i.e., When an X just made an impact of the process)We can ask Minitab to search for 8 out of We can ask Minitab to search for 8 out of control
82、conditions automaticallycontrol conditions automatically2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential2001, General Electric Company, USA. All rights reservedGE ConfidentialGE Confidential X-Bar & Range ChartforWheeler “Master” DataFile: “Wheel_G.mtw”File: “Wh
83、eel_G.mtw”Lets see how we can plot 3 or 4 points.Lets see how we can plot 3 or 4 points.2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialHeres a blank Xbar&R chart (for future use)Heres a blank Xbar&R chart (for future use)2001, General Electric Company, USA. All
84、 rights reservedGE ConfidentialGE ConfidentialThis is how a Control Chart actually looks using MinitabThis is how a Control Chart actually looks using Minitab2001, General Electric Company, USA. All rights reservedGE ConfidentialGE ConfidentialA subtle but crucial distinction: Control limits are not
85、 there to allow us to produce stuff within specifications - they have nothing to do with tolerances in any way. Control limits are there only to allow the process to scream at us when an instability has occurred, in order to get someone off his/her rear-end quickly enough to find-out What just chang
86、ed to cause that? while the tracks remain fresh - before the evidence disappears forever. In this way, the limits help to prevent people from creating additional variation by tampering with a process that should have been left alone, and to keep people from wasting their time and effort looking around for What just changed? when there actually is no rational basis for being suspicious in the first place. Control limits, when established in a rational manner, are an operational definition for When to be suspicious.How to think about SPC:How to think about SPC:
