Figure 1

Control charts of the proportion of inadequate samples across GP practices by Laboratory, before (Jan 2000 – December 2001) and after (October 2005 – September 2006) the introduction of LBC. Circles indicate the proportion of inadequate samples for each practice and the blue lines are the upper and lower control limits. Circles between the control limits indicate practices are displaying common cause variation. Those outside either line are displaying special cause variation. Common cause variation is expected variation attributable to "chance". It is part of every process and affects everyone in that process. To reduce common cause variation we need to fundamentally change the underlying process. In contrast, special cause variation is exceptional variation not attributable to "chance", but arising from special circumstances and therefore not affecting everyone in that process.