AAOM Handbook

INTERNAL

The discipline of Statistical Process Control defines two characteristics of a measure that need to be assessed in order to determine whether an intervention in the process may be required. These are the Stability and the Capability of the process. Stability is an indication of process predictability - is it behaving consistently over time. Capability is an indication of how well the process meets specifications - to what extent do the outputs fit within the specification limits. Control charts and capability histograms of Key Performance Indicators are used to test for the above conditions. The indication that a process is Stable is a Control Chart where;  there aren't unexpected extreme values (outliers) amongst the results, and  the distribution of results is consistent with only random variation occurring in the process, i.e. data points are uncorrelated. The indication that a process is capable is a Capability Histogram where:  the range of the variation in the results is less than the width of the specification limits, and  the results are located within the specification limits. If the above tests are positive then we need to keep doing what we are doing, and any intervention to change that will probably be detrimental to performance (an overreaction). If there is a negative to any of the above four tests, then an action to change what is happening may be required. Adherence to the next steps in the Analyse and Improve will help ensure that the correct action is taken when needed. Investigate Causes In large and complex processes there are likely to be many potential opportunities to improve performance, more than may be practical to pursue at any one time. Not all potential opportunities will provide the same benefit (the Pareto principle suggests that 80% of the benefit will come from 20% of the opportunities), hence, each opportunity should be evaluated objectively and consistently to ensure that we focus on those with the best realisable value. Appropriate criteria for ranking opportunities may include; the potential benefits, cost, ease and timing of delivery and likely sustainability of the change. Recognising an opportunity and its potential doesn’t mean that we understand what needs to be changed in order to realise the opportunity. Some form of investigation of contributing causes will be required to identify exactly what is disturbing the process or inhibiting it meeting specifications. Investigation and analysis should be systematic and objective, as changes based on a false conclusion will at best be a waste of valuable resources, or at worst will cause a harmful change to the process we are trying to correct or improve. Hence, before any action is taken we must measure or test to confirm that the conclusions of the investigation and analysis are valid.

© McAlear Management Consultants 2007

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