Statistical Process Control 5 Part 11 pps

The management of process improvement teams is outside the scope of this book and is dealt with in Total Quality Management Oakland, 2000.. 13.3 Improvements in the process To improve a

For measurements to be used for quality improvement, they must beaccepted by the people involved with the process being measured The simpleself-measurement and plotting, or the ‘how-am-I-doing’ chart, will gain farmore ground in this respect than a policing type of observation and reportingsystem which is imposed on the process and those who operate it Similarly,results should not be used to illustrate how bad one operator or group is,unless their performance is entirely under their own control The emphasis inmeasuring and displaying data must always be on the assistance that can begiven to correct a problem or remove obstacles preventing the process frommeeting its requirements first time, every time.

Out-of-control (OoC) procedures

The rules for interpretation of control charts should be agreed and defined aspart of the SPC system design These largely concern the procedures to befollowed when an out-of-control (OoC) situation develops It is important thateach process ‘operator’ responds in the same way to an OoC indication, and

it is necessary to get their inputs and those of the supervisory management atthe design stage

Clearly, it may not always be possible to define which corrective actionsshould be taken, but the intermediate stage of identifying what happenedshould follow a systematic approach Recording of information, including anysignificant ‘events’, the possible courses of OoC, analysis of causes, and anyaction taken is a vital part of any SPC system design

In some processes, the actions needed to remove or prevent causes of OoCare outside the capability or authority of the process ‘operators’ In thesecases, there must be a mechanism for progressing the preventive actions to becarried out by supervisory management, and their integration into routineprocedures

When improvement actions have been taken on the process, measurementsshould be used to confirm the desired improvements and checks made toidentify any side-effects of the actions, whether they be beneficial ordetrimental It may be necessary to recalculate control chart limits whensufficient data are available, following the changes

Computerized SPC

There are now available many SPC computer software packages which enablethe recording, analysis and presentation of data as charts, graphs and summarystatistics Most of the good ones on the market will readily produce anythingfrom a Pareto diagram to a cusum chart, and calculate skewness, kurtosis andcapability indices They will draw histograms, normal distributions and plots,scatter diagrams and every type of control chart with decision rules included

In using these powerful aids it is, of course, essential that the principles behindthe techniques displayed are thoroughly understood.

13.2 Teamwork and process control/improvement

Teamwork will play a major role in any organization’s efforts to make ending improvements The need for teamwork can be seen in many humanactivities In most organizations, problems and opportunities for improvementexist between departments Seldom does a single department own all themeans to solve a problem or bring about improvement alone

never-Sub-optimization of a process seldom improves the total system ance Most systems are complex, and input from all the relevant processes isrequired when changes or improvements are to be made Teamworkthroughout the organization is an essential part of the implementation of SPC

perform-It is necessary in most organizations to move from a state of independence toone of interdependence, through the following stages:

Little sharing of ideas and information

Exchange of basic information

Exchange of basic ideas

Exchange of feelings and data

Elimination of fearTrustOpen communication

Teamwork brings diverse talents, experience, knowledge and skills to anyprocess situation This allows a variety of problems that are beyond thetechnical competence of any one individual to be tackled Teams can deal withproblems which cross departmental and divisional boundaries All of this ismore satisfying and morale boosting for people than working alone

A team will function effectively only if the results of its meetings arecommunicated and used Someone should be responsible for taking minutes ofmeetings These need not be formal, and simply reflect decisions and actionassignments – they may be copied and delivered to the team members on theway out of the door More formal sets of minutes might be drawn up after the

meetings and sent to sponsors, administrators, supervisors or others who need

to know what happened The purpose of minutes is to inform people ofdecisions made and list actions to be taken Minutes are an important part ofthe communication chain with other people or teams involved in the wholeprocess

Process improvement and ‘Kaisen’ teams

A process improvement team is a group of people with the appropriateknowledge, skills, and experience who are brought together specifically bymanagement to tackle and solve a particular problem, usually on a projectbasis: they are cross-functional and often multi-disciplinary

The ‘task force’ has long been a part of the culture of many organizations

at the technological and managerial levels, but process improvement teams go

a step further, they expand the traditional definition of ‘process’ to include theentire production or operating system This includes paperwork, communica-tion with other units, operating procedures and the process equipment itself

By taking this broader view all process problems can be addressed

The management of process improvement teams is outside the scope of this

book and is dealt with in Total Quality Management (Oakland, 2000) It is

important, however, to stress here the role which SPC techniques themselvescan play in the formation and work of teams For example, the management

in one company, which was experiencing a 17 per cent error rate in its invoicegenerating process, decided to try to draw a flowchart of the process Twopeople who were credited with knowledge of the process were charged withthe task They soon found that it was impossible to complete the flowchart,because they did not fully understand the process Progressively five otherpeople, who were involved in the invoicing, had to be brought to the table inorder that the map could be finished to give a complete description of theprocess This assembled group were kept together as the process improvementteam, since they were the only people who collectively could makeimprovements Simple data collection methods, brainstorming, cause andeffect, and Pareto analysis were then used, together with further flowcharting,

to reduce the error rate to less than 1 per cent within just six months.The flexibility of the cause and effect (C&E) diagram makes it a standardtool for problem solving efforts throughout an organization This simple toolcan be applied in manufacturing, service or administrative areas of a companyand can be applied to a wide variety of problems from simple to very complexsituations

Again the knowledge gained from the C&E diagram often comes from the

method of construction not just the completed diagram A very effective way

to develop the C&E diagram is with the use of a team, representative of thevarious areas of expertise on the effect and processes being studied The C&E

diagram then acts as a collection point for the current knowledge of possiblecauses, from several areas of experience.

Brainstorming in a team is the most effective method of building the C&Ediagram This activity contributes greatly to the understanding, by all thoseinvolved, of a problem situation The diagram becomes a focal point for theentire team and will help any team develop a course for corrective action.Process improvement teams usually find their way into an organization as

problem-solving groups This is the first stage in the creation of problem prevention teams, which operate as common work groups and whose main

objective is constant improvement of processes Such groups may be part of

a multi-skilled, flexible workforce, and include ‘inspect and repair’ tasks aspart of the overall process The so-called ‘Kaisen’ team operates in this way

to eliminate problems at the source by working together and, using very basictools of SPC where appropriate, to create less and less opportunity forproblems and reduce variability Kaisen teams are usually provided with a

‘help line’ which, when ‘pulled’, attracts help from human, technical andmaterial resources from outside the group These are provided specifically forthe purpose of eliminating problems and aiding process control

13.3 Improvements in the process

To improve a process, it is important first to recognize whether the processcontrol is limited by the common or the special causes of variation This will

determine who is responsible for the specific improvement steps, what

resources are required, and which statistical tools will be useful Figure 13.2,

which is a development of the strategy for process improvement presented inChapter 11, may be useful here The comparison of actual product qualitycharacteristics with the requirements (inspection) is not a basis for action onthe process, since unacceptable products or services can result from eithercommon or special causes Product or service inspection is useful to sort outgood from bad and to perhaps set priorities on which processes to improve.Any process left to natural forces will suffer from deterioration, wear andbreakdown (the second law of thermodynamics: entropy is always increas-ing!) Therefore, management must help people identify and prevent thesenatural causes through ongoing improvement of the processes they manage.The organization’s culture must encourage communications throughout andpromote a participative style of management that allows people to reportproblems and suggestions for improvement without fear or intimidation, orenquiries aimed at apportioning blame These must then be addressed withstatistical thinking by all members of the organization

Activities to improve processes must include the assignment of variouspeople in the organization to work on common and special causes The

Figure 13.2 The systematic approach to improvement

appropriate people to identify special causes are usually different to thoseneeded to identify common causes The same is true of those needed toremove causes Removal of common causes is the responsibility ofmanagement, often with the aid of experts in the process such as engineers,chemists and systems analysts Special causes can frequently be handled at

a local level by those working in the process such as supervisors andoperators Without some knowledge of the likely origins of common andspecial causes it is difficult to efficiently allocate human resources toimprove processes

Most improvements require action by management, and in almost all casesthe removal of special causes will make a fundamental change in the wayprocesses are operated For example, a special cause of variation in aproduction process may result when there is a change from one supplier’smaterial to another To prevent this special cause from occurring in theparticular production processes, a change in the way the organization choosesand works with suppliers may be needed Improvements in conformance areoften limited to a policy of single sourcing

Another area in which the knowledge of common and special causes ofvariation is vital is in the supervision of people A mistake often made is theassignment of variation in the process to those working on the process, e.g.operators and staff, rather than to those in charge of the process, i.e.management Clearly, it is important for a supervisor to know whetherproblems, mistakes, or rejected material are a result of common causes,special causes related to the system, or special causes related to the peopleunder his or her supervision Again the use of the systematic approach and theappropriate techniques will help the supervisor to accomplish this

Management must demonstrate commitment to this approach by providingleadership and the necessary resources These resources will include training

on the job, time to effect the improvements, improvement techniques and acommitment to institute changes for ongoing improvement This will movethe organization from having a reactive management system to having one ofprevention This all requires time and effort by everyone, every day

Process control charts and improvements

The emphasis which must be placed on never-ending improvement hasimportant implications for the way in which process control charts are applied.They should not be used purely for control, but as an aid in the reduction ofvariability by those at the point of operation capable of observing andremoving special causes of variation They can be used effectively in theidentification and gradual elimination of common causes of variation

In this way the process of continuous improvement may be charted, andadjustments made to the control charts in use to reflect the improvements

This is shown in Figure 13.3 where progressive reductions in the variability

of ash content in a weedkiller has led to decreasing sample ranges If thecontrol limits on the mean and range charts are recalculated periodically orafter a step change, their positions will indicate the improvements which have

been made over a period of time, and ensure that the new level of process

capability is maintained Further improvements can then take place (Figure13.4) Similarly, attribute or cusum charts may be used, to show a decreasinglevel of number of errors, or proportion of defects and to indicateimprovements in capability

Figure 13.3 Continuous process improvement – reduction in variability

Figure 13.4 Process improvement stages

Often in process control situations, action signals are given when thespecial cause results in a desirable event, such as the reduction of an impuritylevel, a decrease in error rate, or an increase in order intake Clearly, specialcauses which result in deterioration of the process must be investigated andeliminated, but those that result in improvements must also be sought out andmanaged so that they become part of the process operation Significantvariation between batches of material, operators or differences betweensuppliers are frequent causes of action signals on control charts Thecontinuous improvement philosophy demands that these are all investigatedand the results used to take another step on the long ladder to perfection.Action signals and special causes of variation should stimulate enthusiasm forsolving a problem or understanding an improvement, rather than gloom anddespondency.

The never-ending improvement cycle

Prevention of failure is the primary objective of process improvement and iscaused by a management team that is focused on customers The systemwhich will help them achieve ongoing improvement is the so-called Demingcycle (Figure 13.5) This will provide the strategy in which the SPC tools will

be most useful and identify the steps for improvement

Figure 13.5 The Deming cycle

The first phase of the system – plan – helps to focus the effort of theimprovement team on the process The following questions should beaddressed by the team:

customers should be included

one or all of the following:

– eliminate internal difficulties;

– eliminate failure costs;

Every process has many opportunities for improvement, and resources should

be directed to ensure that all efforts will have a positive impact on theobjectives When the objectives of the improvement effort are established,output identified and the customers noted, then the team is ready for theimplementation stage

Implement (Do)

The implementation effort will have the purpose of:

 defining the processes that will be improved;

 identifying and selecting opportunities for improvement

The improvement team should accomplish the following steps duringimplementation:

processes within this system

 Identify the key processes that will contribute to the objectives identified

in the planning stage

 Identify the customer–supplier relationships for the key processes.These steps can be completed by the improvement team through their presentknowledge of the system This knowledge will be advanced throughout theimprovement effort and, with each cycle, the maps/flowcharts and cause andeffect diagrams should be updated The following stages will help the teammake improvements on the selected process:

 Identify and select the process in the system that will offer the greatestopportunities for improvement The team may find that a completedprocess flowchart will facilitate and communicate understanding of theselected process to all team members.

 Document the steps and actions that are necessary to make improvements

It is often useful to consider what the flowchart would look like if everyjob was done right the first time, often called ‘imagineering’

 Define the cause and effect relationships in the process using a cause andeffect diagram

 Identify the important sources of data concerning the process The teamshould develop a data collection plan

 Identify the measurements which will be used for the various parts of theprocess

 Identify the largest contributors to variation in the process The teamshould use their collective experience and brainstorm the possible causes

of variation

During the next phase of the improvement effort, the team will apply theknowledge and understanding gained from these efforts and gain additionalknowledge about the process

Data (Check)

The data collection phase has the following objectives:

implementation phases

 Determine the stability of the process using the appropriate control chartmethod(s)

 If the process is stable, determine the capability of the process

 Prove or disprove any theories established in the earlier phases

determine the impact these will have on the improvement effort

collection adds to current knowledge

Analyse (Act)

The purpose of this phase is to analyse the findings of the prior phases andhelp plan for the next effort of improvement During this phase of processimprovement, the following should be accomplished:

identify the inputs or combinations of inputs that will need to beimproved These should be noted on an updated map of the process

 Develop greater understanding of the causes and effects

 Ensure that the agreed changes have the anticipated impact on thespecified objectives.

analysis, implementation and management of the recommendedchanges

and opportunities discovered in this stage should be considered asobjectives for future efforts Pareto charts should be consulted fromthe earlier work and revised to assist in this process Business processre-design (BPR) may be required to achieve step changes inperformance

Plan, do, check, act (PDCA), as the cycle is often called, will lead toimprovements if it is taken seriously by the team Gaps can occur, however,

in moving from one phase to another unless good facilitation is provided Theteam leader plays a vital role here

13.4 Taguchi methods

Genichi Taguchi has defined a number of methods to simultaneously reducecosts and improve quality The popularity of his approach is a fittingtestimony to the merits of this work The Taguchi methods may be consideredunder four main headings:

 total loss function;

 reduction in variation;

 statistically planned experiments

Total loss function

The essence of Taguchi’s definition of total loss function is that thesmaller the loss generated by a product or service from the time it istransferred to the customer, the more desirable it is Any variation about atarget value for a product or service will result in some loss to thecustomer and such losses should be minimized It is clearly reasonable tospend on quality improvements provided that they result in larger savingsfor either the producer or the customer Earlier chapters have illustratedways in which non-conforming products, when assessed and controlled byvariables, can be reduced to events which will occur at probabilities of theorder of 1 in 100 000 – such reductions will have a large potential impact

on the customer’s losses

Taguchi’s loss function is developed by using a statistical method whichneed not concern us here – but the concept of loss by the customer as ameasure of quality performance is clearly a useful one Figure 13.6 showsthat, if set correctly, a specification should be centred at the position which thecustomer would like to receive all the product This implies that the centre ofthe specification is where the customer’s process works best Product justabove and just below one of the limits is to all intents and purposes the same,

it does not perform significantly differently in the customer’s process and thelosses are unlikely to have the profile shown in (a) The cost of non-conformance is more likely to increase continuously as the actual variableproduced moves away from the centre – as in (b)

Design of products, process and production

For any product or service we may identify three stages of design – theproduct (or service) design, the process (or method) design and the production(or operation) design Each of these overlapping stages has many steps, theoutputs of which are often the inputs to other steps For all the steps, thematching of the outputs to the requirements of the inputs of the next stepclearly affects the quality and cost of the resultant final product or service.Taguchi’s clear classification of these three stages may be used to directmanagement’s effort not only to the three stages but also the separate steps andtheir various interfaces Following this model, management is moved to selectfor study ‘narrowed down’ subjects, to achieve ‘focused’ activity, to increasethe depth of understanding, and to greatly improve the probability of successtowards higher quality levels

(a) Product of MFR 7.1 is

unlikely to work significantly

better than that of 6.9

or

(b) Product at the centre

of the specification is likely

to work better than that at the limits Figure 13.6 Incremental cost ($) of non-conformance

Design must include consideration of the potential problems which willarise as a consequence of the operating and environmental conditions underwhich the product or service will be both produced and used Equally, thecosts incurred during production will be determined by the actual manufactur-ing process Controls, including SPC techniques, will always cost money butthe amount expended can be reduced by careful consideration of controlduring the initial design of the process In these, and many other ways, there

is a large interplay between the three stages of development

In this context, Taguchi distinguishes between ‘on-line’ and ‘off-line’quality management On-line methods are technical aids used for the control

of a process or the control of quality during the production of products andservices – broadly the subject of this book Off-line methods use technical aids

in the design of products and processes Too often the off-line methods arebased on the evaluation of products and processes rather than theirimprovement Effort is directed towards assessing reliability rather than toreviewing the design of both product and process with a view to removingpotential imperfections by design Off-line methods are best directed towardsimproving the capability of design A variety of techniques are possible in thisquality planning activity and include structured teamwork, the use of formalquality/management systems, the auditing of control procedures, the review ofcontrol procedures and failure mode and effect analysis applied on acompany-wide basis

Reduction in variation

Reducing the variation of key processes, and hence product parameters abouttheir target values, is the primary objective of a quality improvementprogramme The widespread practice of stating specifications in terms ofsimple upper and lower limits conveys the idea that the customer is equallysatisfied with all the values within the specification limits and is suddenly notsatisfied when a value slips outside the specification band The practice ofstating a tolerance band may lead to manufacturers aiming to produce anddespatch products whose parameters are just inside the specification band Inany operation, whether mechanical, electrical, chemical, processed food,processed data – as in banking, civil construction, etc – there will be amultiplicity of activities and hence a multiplicity of sources of variation whichall combine to give the total variation

For variables, the mid-specification or some other target value should bestated along with a specified variability about this value For thoseperformance characteristics that cannot be measured on a continuous scale it

is better to employ a scale such as: excellent, very good, good, fair,unsatisfactory, very poor; rather than a simple pass or fail, good or bad

Taguchi introduces a three-step approach to assigning nominal values andtolerances for product and process parameters, as defined in the next threesub-sections.

Design system

The application of scientific, engineering and technical knowledge toproduce a basic functional prototype design requires a fundamental under-standing of both the need of customers and the production possibilities.Trade-offs are not being sought at this stage, but there are requirements for

a clear definition of the customer’s real needs, possibly classified as critical,important and desirable, and an equally clear definition of the supplier’sknown capabilities to respond to these needs, possibly distinguishingbetween the use of existing technology and the development of newtechniques

Parameter design

This entails a study of the whole process system design aimed at achieving themost robust operational settings – those which will react least to variations ofinputs

Process developments tend to move through cycles The most tionary developments tend to start life as either totally unexpected results(fortunately observed and understood) or success in achieving expectedresults, but often only after considerable, and sometimes frustrating, effort.Development moves on through further cycles of attempting to increase thereproducibility of the processes and outputs, and includes the optimization

revolu-of the process conditions to those which are most robust to variations in allthe inputs An ideal process would accommodate wide variations in theinputs with relatively small impacts on the variations in the outputs Someprocesses and the environments in which they are carried out are less prone

to multiple variations than others Types of cereal and domestic animalshave been bred to produce cross-breeds which can tolerate wide variations

in climate, handling, soil, feeding, etc Machines have been designed toallow for a wide range of the physical dimensions of the operators (motorcars, for example) Industrial techniques for the processing of food willaccommodate wide variations in the raw materials with the least influence

on the taste of the final product The textile industry constantly handles, atone end, the wide variations which exist among natural and man-madefibres and, at the other end, garment designs which allow a limited range ofsizes to be acceptable to the highly variable geometry of the human form.Specifying the conditions under which such robustness can be achieved isthe object of parameter design

Tolerance design

A knowledge of the nominal settings advanced by parameter design enablestolerance design to begin This requires a trade-off between the costs ofproduction or operation and the losses acceptable to the customer arising fromperformance variation It is at this stage that the tolerance design of cars orclothes ceases to allow for all versions of the human form, and that eitherblandness or artificial flavours may begin to dominate the taste of processedfood

These three steps pass from the original concept of the potential for aprocess or product, through the development of the most robust conditions ofoperation, to the compromise involved when setting ‘commercial’ tolerances– and focus on the need to consider actual or potential variations at all stages.When considering variations within an existing process it is clearly beneficial

to similarly examine their contributions from the three points of view

Statistically planned experiments

Experimentation is necessary under various circumstances and in particular inorder to establish the optimum conditions which give the most robust process– to assess the parameter design ‘Accuracy’ and ‘precision’, as defined inChapter 5, may now be regarded as ‘normal settings’ (target or optimumvalues of the various parameters of both processes and products) and ‘noise’(both the random variation and the ‘room’ for adjustment around the nominalsetting) If there is a problem it will not normally be an unachievable nominalsetting but unacceptable noise Noise is recognized as the combination of therandom variations and the ability to detect and adjust for drifts of the nominalsetting Experimentation should, therefore, be directed towards maintainingthe nominal setting and assessing the associated noise under variousexperimental conditions Some of the steps in such research will already befamiliar to the reader These include grouping data together, in order to reducethe effect on the observations of the random component of the noise andexposing more readily the effectiveness of the control mechanism, theidentification of special causes, the search for their origins and the evaluation

of individual components of some of the sources of random variation.Noise is divided into three classes, outer, inner and between Outer noiseincludes those variations whose sources lie outside the management’scontrols, such as variations in the environment which influence the process(for example, ambient temperature fluctuations) Inner noise arises fromsources which are within management’s control but not the subject of thenormal routine for process control, such as the condition or age of a machine.Between noise is that tolerated as a part of the control techniques in use – this

is the ‘room’ needed to detect change and correct for it Trade-off betweenthese different types of noise is sometimes necessary Taguchi quotes the case

of a tile manufacturer who had invested in a large and expensive kiln forbaking tiles, and in which the heat transfer through the oven and the resultanttemperature cycle variation gave rise to an unacceptable degree of productvariation Whilst a redesign of the oven was not impossible, both cost and timemade this solution unavailable – the kiln gave rise to ‘outer’ noise Effort had,therefore, to be directed towards finding other sources of variation, either

‘inner’ or ‘between’, and, by reducing the noise they contributed, bringing thetotal noise to an acceptable level It is only at some much later date, whenspecifying the requirements of a new kiln, that the problem of the outer noisebecomes available and can be addressed

In many processes, the number of variables which can be the subject ofexperimentation is vast, and each variable will be the subject of a number ofsources of noise within each of the three classes So the possible combinationsfor experimentation is seemingly endless The ‘statistically planned experi-ment’ is a system directed towards minimizing the amount of experimentation

to yield the maximum of results and in doing this to take account of bothaccuracy and precision – nominal settings and noise Taguchi recognized that

in any ongoing industrial process the list of the major sources of variation andthe critical parameters which are affected by ‘noise’ are already known So thecombination of useful experiments may be reduced to a manageable number

by making use of this inherent knowledge Experimentation can be used toidentify:

parameters and/or performance;

 the design parameters which have no influence on the product or processperformance characteristics;

 the setting of design parameters at levels which minimize the noise withinthe performance characteristics;

adversely affecting cost

As with nearly all the techniques and facets of SPC, the ‘design ofexperiments’ is not new; Tippet used these techniques in the textile industrymore than 50 years ago Along with the other quality gurus, Taguchi hasenlarged the world’s view of the applications of established techniques Hismajor contributions are in emphasizing the cost of quality by use of the totalloss function and the sub-division of complex ‘problem solving’ intomanageable component parts The author hopes that this book will make asimilar, modest, contribution towards the understanding and adoption ofunder-utilized process management principles

13.5 Summarizing improvement

Improving products or service quality is achieved through improvements inthe processes that produce the product or the service Each activity and eachjob is part of a process which can be improved Improvement is derivedfrom people learning and the approaches presented above provide a ‘roadmap’ for progress to be made The main thrust of the approach is a teamwith common objectives – using the improvement cycle, defining currentknowledge, building on that knowledge, and making changes in the process.Integrated into the cycle are methods and tools that will enhance thelearning process

When this strategy is employed, the quality of products and services isimproved, job satisfaction is enhanced, communications are strengthened,productivity is increased, costs are lowered, market share rises, new jobs areprovided and additional profits flow In other words, process improvement

as a business strategy provides rewards to everyone involved: customersreceive value for their money, employees gain job security, and owners orshareholders are rewarded with a healthy organization capable of payingreal dividends This strategy will be the common thread in all companieswhich remain competitive in world markets in the twenty-first century

Chapter highlights

quality policy and a documented management system

reduction in variation in processes The system should apply to andinteract with all activities of the organization

procedures and the methods The system audit and review will ensure theprocedures are followed or changed

activities, materials, equipment, etc., to be measured must be identifiedprecisely The measurements must be accepted by the people involvedand, in their use, the emphasis must be on providing assistance to solveproblems

followed when out-of-control (OoC) situations develop should be agreedand defined as part of the SPC system design

organizations it means moving from ‘independence’ to dependence’ Inputs from all relevant processes are required to make