David S Simpson, Tony Roberts, Chris Walker, Kevin D Cooper, Fiona O?Brien
Healthcare process measurements are routinely completed in the NHS. Statistical Process Control (SPC) techniques, when applied to such data, can be used as a basis for quality improvement in healthcare processes in much the same way as they have been effectively applied to manufacturing. WA Shewhart recognised in the 1920s that a process can contain two types of variation – that due to random causes and that due to assignable causes (i.e. random (common) or assignable (special)). WE Deming later derived the expressions ‘common cause variation’ and ‘special cause variation’ – common cause variation is an inherent part of all processes; that is, it is ever present. Special cause variation is that due to things that really weren’t part of the way the process was designed, and which somehow artificially find their way into it. Since a reason for its presence can be identified, its effect on the process is usually infrequent and can often be eliminated, but the effect on outcomes can be huge. Healthcare data should be used to guide quality improvements, and the role of SPC in this process is to identify assignable (special) causes and understand its origin (it should be prevented if bad and spread if good). This graphically informative approach of presenting health data is an alternative method to conventionalthings such as performance league tables for presenting outcomes, because tables with only common cause variation tend to encourage unwarranted tampering, may lead to local special cause variation being ignored, tend to encourage the ‘blame culture’, and are not linked directly to improvement activity.
