The number of defects per unit (u) chart is used to monitor the average number of defects per item in samples which may vary in size from one sample to the next.
Each sample examined should not vary by more than ±20% from the target sample size specified in the chart headings. If the size of any sample goes beyond either the upper or lower limit, the control limits must be re-calculated for that sample. If each sample stays within this range, the same set of control limits can be used for plotting the whole of the chart.
A u-Chart for Multiple Characteristics allows you to record more detailed information about the particular characteristics which are classified as defects. This information can be used as the basis for pareto analysis.
The formula required to calculate the values to be plotted on the chart is:
u = c / n
When it has been calculated, the value of u is recorded in the data recording boxes and plotted on the u-graph.