TABLE OF CONTENTS This chapter is concerned with two aspects of laboratory quality control, viz., control charting, which is a form of internal quality control, and proficiency testing, which is a form of external quality control. The main focus is on control charts, as they are the simplest and cheapest method for obtaining assurance that the laboratory's analytical systems are well-behaved and, hence, that the data routinely produced by the laboratory are fit for purpose.
Statistical quality control charts are simple but powerful tools for monitoring the stability of analytical systems. A control material is measured regularly and the analytical responses are plotted in time order on a chart; if the chart displays other than random variation around the expected result it suggests that something has gone wrong with the measurement process. To help decide if this has happened control limits are plotted on the chart: the responses are expected to remain inside these limits. Rules are decided upon which will define non-random behaviour.
Control charts were developed to control engineering production processes rather than measurement systems. However, any apparent differences between the two situations disappear when we think of a measurement system as a production process whose output is measurements. The most commonly used charts were developed by Shewhart1 in Bell Laboratories in the 1920s. In describing factors that affect any system he distinguished between 'chance causes' and 'assignable causes'. In an analytical context, chance causes are the myriad of small influences that lead to the Normal...
