NP Chart

The NP chart is one of the most intuitive and accessible control charts for attribute data. It monitors the number of defective units in a sample, making it ideal for processes where each unit is classified simply as “defective” or “not defective.” Unlike the P chart, which tracks proportions, the NP chart tracks counts—but only when the sample size is constant. This simplicity makes it a powerful tool in the Control phase for teams who need a clear, actionable view of quality performance. 

The NP chart begins with a fixed sample size—say, inspecting 100 units per batch. Each sample produces a count of defective units. These counts are plotted over time, along with a center line representing the average number of defectives and control limits that reflect expected variation. Because the sample size is constant, the control limits remain constant as well, making the chart easy to interpret. 

One of the strengths of the NP chart is its clarity. Operators and frontline teams often find it easier to understand counts than proportions. A spike in defectives is immediately visible. A run of low counts signals improvement. The chart provides a direct, intuitive view of quality performance without requiring calculations or conversions. 

However, the NP chart requires discipline in sampling. If the sample size varies—even slightly—the chart becomes invalid. This means that the sampling plan must be stable, consistent, and well‑defined. If the process cannot support consistent sample sizes, a P chart is more appropriate. 

Interpreting an NP chart involves looking for points outside the control limits, runs of increasing or decreasing counts, or patterns that suggest special causes. A point above the upper control limit indicates a spike in defects that requires investigation. A point below the lower control limit may indicate improvement—but only if the process is stable and the change is sustained. 

In the Control phase, the NP chart provides a simple, powerful way to monitor defect counts, maintain stability, and sustain improvements. Its clarity makes it ideal for frontline ownership and daily management.