Attribute study type

An attribute chart is a type of control chart that measures attribute data against continuous data. Your choice of charts depends on whether you have a problem with defects or defectives, and whether you have a fixed or varying sample size.

NP chart

An 'NP' attribute study type measures the number of failing pieces from a consistently fixed sample size. This means every time an inspection is performed it is always against a specified number of pieces and the total number of failing pieces is counted. Only the attribute/defect determined to be the reason for the piece's failure is recorded. The total number of defects cannot exceed the sample size in this mode.

P chart

A 'P' attribute study type measures the number of failing pieces from a varying sample size. This means each time an inspection is performed the number of pieces might vary, and it is the total number of failing pieces in this set that is counted. Only the attribute/defect determined to be the reason for the piece's failure is recorded. The total number of defects cannot exceed the sample size in this mode.

C chart

A 'C' attribute study type counts every defect associated with a consistently fixed sample size. This means every time an inspection is performed it is always against a specified number of pieces but every defect is counted for the entire set. The total number of defects can exceed the sample size in this mode.

U chart

A 'U' attribute study type counts every defect associated with a varying sample size. This means each time an inspection is performed the number of pieces might vary but every defect is counted for the entire set. The total number of defects can exceed the sample size in this mode.