Statistical Process Control 5 Part 11 pps

Designing the SPC system 337 For measurements to be used for quality improvement, they must be accepted by the people involved with the process being measured. The simple self-measurement and plotting, or the ‘how-am-I-doing’ chart, will gain far more ground in this respect than a policing type of observation and reporting system 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 in measuring and displaying data must always be on the assistance that can be given to correct a problem or remove obstacles preventing the process from meeting its requirements first time, every time. Out-of-control (OoC) procedures The rules for interpretation of control charts should be agreed and defined as part of the SPC system design. These largely concern the procedures to be followed when an out-of-control (OoC) situation develops. It is important that each 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 at the design stage. Clearly, it may not always be possible to define which corrective actions should be taken, but the intermediate stage of identifying what happened should follow a systematic approach. Recording of information, including any significant ‘events’, the possible courses of OoC, analysis of causes, and any action taken is a vital part of any SPC system design. In some processes, the actions needed to remove or prevent causes of OoC are outside the capability or authority of the process ‘operators’. In these cases, there must be a mechanism for progressing the preventive actions to be carried out by supervisory management, and their integration into routine procedures. When improvement actions have been taken on the process, measurements should be used to confirm the desired improvements and checks made to identify any side-effects of the actions, whether they be beneficial or detrimental. It may be necessary to recalculate control chart limits when sufficient data are available, following the changes. Computerized SPC There are now available many SPC computer software packages which enable the recording, analysis and presentation of data as charts, graphs and summary statistics. Most of the good ones on the market will readily produce anything from a Pareto diagram to a cusum chart, and calculate skewness, kurtosis and capability indices. They will draw histograms, normal distributions and plots, scatter diagrams and every type of control chart with decision rules included. 338 Designing the SPC system In using these powerful aids it is, of course, essential that the principles behind the 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 never- ending improvements. The need for teamwork can be seen in many human activities. In most organizations, problems and opportunities for improvement exist between departments. Seldom does a single department own all the means to solve a problem or bring about improvement alone. Sub-optimization of a process seldom improves the total system perform- ance. Most systems are complex, and input from all the relevant processes is required when changes or improvements are to be made. Teamwork throughout the organization is an essential part of the implementation of SPC. It is necessary in most organizations to move from a state of independence to one 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 fear Trust Open communication       ↓ TIME The communication becomes more open with each progressive step in a successful relationship. The point at which it increases dramatically is when trust is established. After this point, the barriers that have existed are gone and open communication will proceed. This is critical for never-ending improve- ment and problem solving, for it allows people to supply good data and all the facts without fear. Teamwork brings diverse talents, experience, knowledge and skills to any process situation. This allows a variety of problems that are beyond the technical competence of any one individual to be tackled. Teams can deal with problems which cross departmental and divisional boundaries. All of this is more satisfying and morale boosting for people than working alone. A team will function effectively only if the results of its meetings are communicated and used. Someone should be responsible for taking minutes of meetings. These need not be formal, and simply reflect decisions and action assignments – they may be copied and delivered to the team members on the way out of the door. More formal sets of minutes might be drawn up after the Designing the SPC system 339 meetings and sent to sponsors, administrators, supervisors or others who need to know what happened. The purpose of minutes is to inform people of decisions made and list actions to be taken. Minutes are an important part of the communication chain with other people or teams involved in the whole process. Process improvement and ‘Kaisen’ teams A process improvement team is a group of people with the appropriate knowledge, skills, and experience who are brought together specifically by management to tackle and solve a particular problem, usually on a project basis: 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 the entire 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 themselves can 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 invoice generating process, decided to try to draw a flowchart of the process. Two people who were credited with knowledge of the process were charged with the task. They soon found that it was impossible to complete the flowchart, because they did not fully understand the process. Progressively five other people, who were involved in the invoicing, had to be brought to the table in order that the map could be finished to give a complete description of the process. This assembled group were kept together as the process improvement team, since they were the only people who collectively could make improvements. Simple data collection methods, brainstorming, cause and effect, 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 standard tool for problem solving efforts throughout an organization. This simple tool can be applied in manufacturing, service or administrative areas of a company and can be applied to a wide variety of problems from simple to very complex situations. 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 the various areas of expertise on the effect and processes being studied. The C&E 340 Designing the SPC system diagram then acts as a collection point for the current knowledge of possible causes, from several areas of experience. Brainstorming in a team is the most effective method of building the C&E diagram. This activity contributes greatly to the understanding, by all those involved, of a problem situation. The diagram becomes a focal point for the entire 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 as part 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 basic tools of SPC where appropriate, to create less and less opportunity for problems and reduce variability. Kaisen teams are usually provided with a ‘help line’ which, when ‘pulled’, attracts help from human, technical and material resources from outside the group. These are provided specifically for the 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 process control 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 in Chapter 11, may be useful here. The comparison of actual product quality characteristics with the requirements (inspection) is not a basis for action on the process, since unacceptable products or services can result from either common or special causes. Product or service inspection is useful to sort out good from bad and to perhaps set priorities on which processes to improve. Any process left to natural forces will suffer from deterioration, wear and breakdown (the second law of thermodynamics: entropy is always increas- ing!). Therefore, management must help people identify and prevent these natural causes through ongoing improvement of the processes they manage. The organization’s culture must encourage communications throughout and promote a participative style of management that allows people to report problems and suggestions for improvement without fear or intimidation, or enquiries aimed at apportioning blame. These must then be addressed with statistical thinking by all members of the organization. Activities to improve processes must include the assignment of various people in the organization to work on common and special causes. The Designing the SPC system 341 Figure 13.2 The systematic approach to improvement 342 Designing the SPC system appropriate people to identify special causes are usually different to those needed to identify common causes. The same is true of those needed to remove causes. Removal of common causes is the responsibility of management, 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 and operators. Without some knowledge of the likely origins of common and special causes it is difficult to efficiently allocate human resources to improve processes. Most improvements require action by management, and in almost all cases the removal of special causes will make a fundamental change in the way processes are operated. For example, a special cause of variation in a production process may result when there is a change from one supplier’s material to another. To prevent this special cause from occurring in the particular production processes, a change in the way the organization chooses and works with suppliers may be needed. Improvements in conformance are often limited to a policy of single sourcing. Another area in which the knowledge of common and special causes of variation is vital is in the supervision of people. A mistake often made is the assignment 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 whether problems, mistakes, or rejected material are a result of common causes, special causes related to the system, or special causes related to the people under his or her supervision. Again the use of the systematic approach and the appropriate techniques will help the supervisor to accomplish this. Management must demonstrate commitment to this approach by providing leadership and the necessary resources. These resources will include training on the job, time to effect the improvements, improvement techniques and a commitment to institute changes for ongoing improvement. This will move the organization from having a reactive management system to having one of prevention. 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 has important 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 of variability by those at the point of operation capable of observing and removing special causes of variation. They can be used effectively in the identification and gradual elimination of common causes of variation. In this way the process of continuous improvement may be charted, and adjustments made to the control charts in use to reflect the improvements. Designing the SPC system 343 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 the control limits on the mean and range charts are recalculated periodically or after 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 (Figure 13.4). Similarly, attribute or cusum charts may be used, to show a decreasing level of number of errors, or proportion of defects and to indicate improvements in capability. Figure 13.3 Continuous process improvement – reduction in variability Figure 13.4 Process improvement stages 344 Designing the SPC system Often in process control situations, action signals are given when the special cause results in a desirable event, such as the reduction of an impurity level, a decrease in error rate, or an increase in order intake. Clearly, special causes which result in deterioration of the process must be investigated and eliminated, but those that result in improvements must also be sought out and managed so that they become part of the process operation. Significant variation between batches of material, operators or differences between suppliers are frequent causes of action signals on control charts. The continuous improvement philosophy demands that these are all investigated and the results used to take another step on the long ladder to perfection. Action signals and special causes of variation should stimulate enthusiasm for solving a problem or understanding an improvement, rather than gloom and despondency. The never-ending improvement cycle Prevention of failure is the primary objective of process improvement and is caused by a management team that is focused on customers. The system which will help them achieve ongoing improvement is the so-called Deming cycle (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 Designing the SPC system 345 Plan The first phase of the system – plan – helps to focus the effort of the improvement team on the process. The following questions should be addressed by the team: ᭹ What are the requirements of the output from the process? ᭹ Who are the customers of the output? Both internal and external customers should be included. ᭹ What are the objectives of the improvement effort? These may include one or all of the following: – improve customer satisfaction; – eliminate internal difficulties; – eliminate unnecessary work; – eliminate failure costs; – eliminate non-conforming output. Every process has many opportunities for improvement, and resources should be directed to ensure that all efforts will have a positive impact on the objectives. When the objectives of the improvement effort are established, output identified and the customers noted, then the team is ready for the implementation 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 during implementation: ᭹ Define the scope of the system to be improved and map or flowchart the 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 present knowledge of the system. This knowledge will be advanced throughout the improvement effort and, with each cycle, the maps/flowcharts and cause and effect diagrams should be updated. The following stages will help the team make improvements on the selected process: 346 Designing the SPC system ᭹ Identify and select the process in the system that will offer the greatest opportunities for improvement. The team may find that a completed process flowchart will facilitate and communicate understanding of the selected 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 every job was done right the first time, often called ‘imagineering’. ᭹ Define the cause and effect relationships in the process using a cause and effect diagram. ᭹ Identify the important sources of data concerning the process. The team should develop a data collection plan. ᭹ Identify the measurements which will be used for the various parts of the process. ᭹ Identify the largest contributors to variation in the process. The team should use their collective experience and brainstorm the possible causes of variation. During the next phase of the improvement effort, the team will apply the knowledge and understanding gained from these efforts and gain additional knowledge about the process. Data (Check) The data collection phase has the following objectives: ᭹ To collect data from the process as determined in the planning and implementation phases. ᭹ Determine the stability of the process using the appropriate control chart method(s). ᭹ If the process is stable, determine the capability of the process. ᭹ Prove or disprove any theories established in the earlier phases. ᭹ If the team observed any unplanned events during data collection, determine the impact these will have on the improvement effort. ᭹ Update the maps/flowcharts and cause and effect diagrams, so the data collection adds to current knowledge. Analyse (Act) The purpose of this phase is to analyse the findings of the prior phases and help plan for the next effort of improvement. During this phase of process improvement, the following should be accomplished: ᭹ Determine the action on the process which will be required. This will identify the inputs or combinations of inputs that will need to be improved. These should be noted on an updated map of the process. ᭹ Develop greater understanding of the causes and effects. . eliminating problems and aiding process control. 13.3 Improvements in the process To improve a process, it is important first to recognize whether the process control is limited by the common. (1993) Statistical Process Control for Quality Improvement, Kluwer, Netherlands. Designing the SPC system 355 Discussion questions 1 Explain how a documented management system can help to reduce process variation of the normal 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 352 Designing the SPC system is