# Dr Wheeler weighs in on "Individual Charts Done Right and Wrong"

#### bobdoering

Trusted Information Resource
Dr Wheeler's article discusses the calculations for XmR charting:Individual Charts Done Right and Wrong He notes that some software will allow you to pick algorithms that calculate control limits erroneously. My favorite quote from this article: "Thus, just as with average charts, there are right and wrong ways to compute limits for a chart for individual values. Because most software will provide you with the option of computing your limits incorrectly, it is your responsibility to determine how to get the right limits using your software." I might add for some charting, the correct control limit methodology is not available in the software at all, and you are on your own.

#### Stijloor

Super Moderator
Dr Wheeler's article discusses the calculations for XmR charting:Individual Charts Done Right and Wrong He notes that some software will allow you to pick algorithms that calculate control limits erroneously. My favorite quote from this article: "Thus, just as with average charts, there are right and wrong ways to compute limits for a chart for individual values. Because most software will provide you with the option of computing your limits incorrectly, it is your responsibility to determine how to get the right limits using your software." I might add for some charting, the correct control limit methodology is not available in the software at all, and you are on your own.

Great article Bob! Thanks for sharing.

Stijloor.

#### Steve Prevette

##### Deming Disciple
Super Moderator
I do think that the "problems" with using the statistical estimate of the standard deviation need to be revisited. Some things to consider:

1. The example provided by Dr. Wheeler has a run of 7 points in a row below average, which by many authors (but not Dr. Wheeler, who uses 8 points) to be a signal, which would cause that data not to be put all into the same baseline.

2. In 16 years of operational SPC, I have yet to run into an example from real world data where the moving range and the statistical standard deviation provide two different interpretations of what is and what is not an outlier.

3. With modern spreadsheets, the statistical standard deviation is easier to calculate rather than the moving range conversion. That is the opposite of the situation for pre-computer days, when the moving range was developed.

4. Dr. Shewhart himself, in Economic Control of Quality of Manufactured Product evaluates several methods for determining the spread of the data, and he rates the statistical standard deviation as best (better than the range).

5. Dr. Shewhart and other authors do invoke the Tchybychev Inequality as the theoretical basis for SPC. The Tchybychev Inequality is non-parametric and uses the statistical estimate for the standard deviation. The moving range formula (2.66 times the average moving range) has the Normal distribution built into it as an assumption.

Dr. Wheeler's series on Data Snooping (aka Data Mining) Statistical Analysis Tools, Techniques and SPC 1
S GR&R by MSA Rules vs. Wheeler & Lyday's EMP Method Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 8
Dr. Wheeler article: What About p-Charts? - When should we use the specialty charts Statistical Analysis Tools, Techniques and SPC 2
P Analysis of the figures from Dr Wheeler's 'Short Run SPC' Using Minitab Software 3
Dr. Wheeler: Myths About Process Behavior Charts Statistical Analysis Tools, Techniques and SPC 3
Dr Wheeler gives his thoughts on "Estimating the Fraction Nonconforming" Capability, Accuracy and Stability - Processes, Machines, etc. 0
Dr. Wheeler on "How Measurement Error Affects the Four Ways We Use Data" Measurement Uncertainty (MU) 0
Dr. Wheeler is back with his thoughts on MSA and Gage R&R Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 2
Hero Honda No More - The Two-Wheeler Dominated Market World News 1
B Wheeler Honest Gauge R&R macro wanted Excel .xls Spreadsheet Templates and Tools 2
R Is TS 16949 applicable to Two Wheeler manufacturing companies? IATF 16949 - Automotive Quality Systems Standard 4
S Gage R&R - Wheeler's use of 0.675 x Sigma (equip. Var.) as the "Probable Error" Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 5
Dr Wheeler pokes at the Theorem of Large Numbers Statistical Analysis Tools, Techniques and SPC 2
Quick Overview of SPC Scaling Factors from Wheeler Statistical Analysis Tools, Techniques and SPC 0
An Honest Gauge R&R Analysis - Proposed usage of Donald Wheeler Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 15
Wheeler is back again! He says it is the last volley! Statistical Analysis Tools, Techniques and SPC 7
Wheeler's back - Railing against Transformations Statistical Analysis Tools, Techniques and SPC 7
Intro to MSA of Continuous Data – Part 7: R&R using Wheeler’s Honest Gage Study Imported Legacy Blogs 17
Dr. Wheeler takes a shot at Normal-centrics Statistical Analysis Tools, Techniques and SPC 1
P A collection of Dr. Wheeler's Articles from SPC Toolkit column of Quality Digest Statistical Analysis Tools, Techniques and SPC 3
P Dr. Wheeler's not using p-chart for countable events? Statistical Analysis Tools, Techniques and SPC 6
Don Wheeler article on MSA - Good Data, Bad Data and Process Behavior Charts Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 7
S Can a "Two Wheeler" Manufacturer Supplier Register? IATF 16949 - Automotive Quality Systems Standard 7
Cpk in relation to individual process parameters. Reliability Analysis - Predictions, Testing and Standards 1
The FDA regulations (21 CFR 312.3): Is it allowable that IND sponsor involves more than one individual or organization? Pharmaceuticals (21 CFR Part 210, 21 CFR Part 211 and related Regulations) 0
Change color or shape of individual data point in control chart Using Minitab Software 6
Records Control - Does each individual record need to be numbered? Records and Data - Quality, Legal and Other Evidence 2
EU Shelf life claims for individual materials EU Medical Device Regulations 2
P Large data sets of continuous individual data - Estimated or actual deviation Capability, Accuracy and Stability - Processes, Machines, etc. 2
Can an individual apply for an MDEL in Canada, as an establishment? Canada Medical Device Regulations 6
G How should we document individual Support Processes ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 1
N Process Capability Study - Aggregate vs. Individual Processes Statistical Analysis Tools, Techniques and SPC 27
G Labeling requirements for individual units for medical devices 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 2
N Injection Molding - Median individual control charts Statistical Analysis Tools, Techniques and SPC 6
J Individual part traceability - ink/paint sprayer? IATF 16949 - Automotive Quality Systems Standard 4
Tracking response time with individual label I.D. Statistical Analysis Tools, Techniques and SPC 6
B Cleanroom Control Limits: Cumulative or individual? Statistical Analysis Tools, Techniques and SPC 11
I Dual Acceptance Criteria: Cpk and Individual Values Statistical Analysis Tools, Techniques and SPC 3
Mix Multi-Site/Individual Certification - Has this ever happened to anyone? Miscellaneous Environmental Standards and EMS Related Discussions 14
M Individual Assembly Routing Question Manufacturing and Related Processes 4
1 VBA Needed to Protect Individual Excel Cells Excel .xls Spreadsheet Templates and Tools 2
K Equipment Lifetime: Put in operation manual or a individual document/declaration Quality Manager and Management Related Issues 1
T Implementing SPC - Control Chart for Individual Measurements Statistical Analysis Tools, Techniques and SPC 9
9001 Corporate Quality Audit - Yearly Audit on the Individual Business Units General Auditing Discussions 4
N Capability on Parallel Processes such as Individual Mould Cavities Philosophy, Gurus, Innovation and Evolution 22
P Responsibilities of Individual named as Submitter of 510(k) 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 4
B Color Individual Points on a Control Chart? Using Minitab Software 7
P 510K Help for Individual Medical Device Inventor 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 5
K If an individual requests my ISO 9001 QMS Documents, do I have to supply it? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 7
B IMR or Individual Run SPC Charts In Minitab Using Minitab Software 3