Data - Capability Analysis (Poisson)
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Each entry in the worksheet column should contain the number of defects for a subgroup. When subgroup sizes are unequal, you must also enter a corresponding column of subgroup sizes.

Suppose you have collected daily data on the number of parts that have been inspected and the number of defects. On any given day both numbers may vary. You enter the number of defects in one column. In this case, the total number inspected varies from day to day, so you enter the subgroup size in another column.

Missing Data

If an observation is missing, there is a gap in the U chart where the subgroup would have been plotted. The other plots and charts simply exclude the missing observation(s).

Unequal Subgroup Sizes

In the U chart, the control limits are a function of the subgroup size. In general, the control limits are further from the centerline for smaller subgroups than they are for larger ones. When you do have unequal subgroup sizes, the plot of defects per unit (DPU) versus sample size will permit you to verify that there is no relationship between the two. For example, if you tend to have a smaller DPU when more items are sampled, fatigued inspectors could cause this, which is a common problem. The subgroup size has no bearing on the other charts, because they only display the DPU.