Internal analytical quality control using the cusum chart and truncated V-mask procedure.

It is recommended that quality control data be recorded on a standard form to facilitate calculation of the cusum increments and of the cumulative sums (cusums) for plotting on the cusum chart. The 'in control' and 'out of control' judgements are made with the aid of a truncated V-mask. Full details of calculation procedures are described without recourse to statistical theory so that the cusum and truncated V-mask procedure may be carried out by operators without training in statistics.