P-, NP-, U- and C- charts are tools used in quality management to monitor and control processes. These tools are often used within the statistical process control method (SPC – Statistical Process Control ).
- A P- chart is used to track and manage the percentage of defective items in a process. It is used when the amount of products being produced is constant and the same for each sample.
- The NP- chart is used to track and manage the number of defective items in the process. Compared to the P-chart, the NP- chart is used in cases where the sample size is the same in each monitored section of the process, but the amount of products may differ.
- A U- chart is a tool used to track and control the number of defective units in a process where the sample size may vary and is not constant.
- A C- chart is used to track and control the number of defective elements. In a process where the sample size is constant and each element may have multiple defective units (for example, defective points on the surface of the product).
These tools in SPC are used to monitor processes, identify trends, deviations and possible crossing of control limits. They assist managers and operators in deciding whether a process is under control. Or whether corrective action needs to be taken to improve process quality.