The c-Chart is also known as the Number of Defects or Number of Non-Conformities Chart. For a sample subgroup (also called the inspection unit), the number of times a defect occurs is measured and plotted as a simple count. Statistically, in order to compare number of defects for one subgroup with the other subgroups, this type of chart requires that the subgroup sample size is fixed across all subgroups.
Note that this chart tracks the number of defects, not the number of defective parts as done in the p-chart, and np-chart. For any give part, you can have 0 to N defects. Defects are things like scratches, dents, chips, paint flaws, etc.; think of the last car you bought. Also, a defect does not indicate any magnitude of defect (such as might be measured in one of the variable control charts), only that it is, or is not a defect. If you were monitoring a process using both p-charts and c-charts, the p-chart may show that 55 parts were defective, while the c-chart shows that 175 defects were present, since a single part can have one or more defects.
c-Chart – 1