a. Select the subgroup size (n). Attributes data often require large subgroup sizes (50 - 200). The subgroup size should be large enough to have several defective items. The subgroup size must be constant.
In the red bead example, the subgroup size is 50.
b. Select the frequency with which the data will be collected. Data should be collected in the order in which it is generated.
c. Select the number of subgroups (k) to be collected before control limits are calculated. You can start a control chart with as few as five to six points but you should recalculate the average and control limits until you have about 20 subgroups.
d. Inspect each item in the subgroup and record the item as either defective or non-defective. If an item has several defects, it is still counted as one defective item.
e. Determine np for each subgroup.