An NP Chart is a statistical process control tool used in the Control Phase of Lean Six Sigma to monitor the number of defective items in a process when the sample size remains constant. The 'NP' stands for 'number of defectives times probability,' making it ideal for attribute data where items are…An NP Chart is a statistical process control tool used in the Control Phase of Lean Six Sigma to monitor the number of defective items in a process when the sample size remains constant. The 'NP' stands for 'number of defectives times probability,' making it ideal for attribute data where items are classified as either conforming or non-conforming.
This chart is particularly useful when you want to track the total count of defective units rather than the proportion of defects. For example, if you inspect 100 units each day and count how many fail quality standards, an NP Chart would be appropriate for visualizing this data over time.
The NP Chart consists of three key lines: the center line (CL), which represents the average number of defectives; the upper control limit (UCL); and the lower control limit (LCL). These control limits are typically set at three standard deviations from the mean, capturing approximately 99.7% of expected variation.
The formulas for an NP Chart are straightforward. The center line equals np-bar (the average number of defectives). The UCL is calculated as np-bar plus three times the square root of np-bar times (1-p-bar). The LCL uses the same formula but subtracts instead of adds.
When using an NP Chart, practitioners plot each sample's defective count and look for patterns indicating special cause variation. Points falling outside control limits, runs of seven or more points on one side of the center line, or trending patterns all signal potential process issues requiring investigation.
The main advantage of NP Charts over P Charts is simplicity in interpretation since actual counts are displayed rather than proportions. However, NP Charts require consistent sample sizes. If your sample sizes vary, a P Chart would be more appropriate. NP Charts serve as valuable tools for maintaining process stability during the Control Phase and ensuring improvements achieved during earlier DMAIC phases are sustained.
NP Chart: Complete Guide for Six Sigma Green Belt Control Phase
What is an NP Chart?
An NP Chart (Number of Defective Units Chart) is a statistical process control tool used to monitor the number of defective items in samples of constant size. It is classified as an attribute control chart because it deals with discrete data—items are either defective or not defective.
The 'N' represents the sample size, and 'P' represents the proportion defective. The NP chart plots the actual count of defective units rather than the proportion.
Why is the NP Chart Important?
• Process Monitoring: It helps organizations track the stability of processes over time by identifying when the number of defects exceeds acceptable limits.
• Early Detection: NP charts enable teams to detect shifts or trends in defect rates before they become critical problems.
• Data-Driven Decisions: They provide objective evidence for process improvement initiatives and help validate whether changes have been effective.
• Simplicity: Compared to P charts, NP charts are easier to interpret because they display actual counts rather than proportions.
How Does an NP Chart Work?
Prerequisites: • Sample size (n) must be constant across all samples • Data must be attribute data (pass/fail, defective/non-defective) • Each unit is independent
Key Formulas:
• Center Line (NP-bar): NP-bar = Total number of defectives ÷ Number of samples
Note: If LCL calculates to a negative value, set it to zero.
Interpretation: • Points within control limits indicate a stable process • Points outside control limits signal special cause variation • Patterns such as runs, trends, or cycles suggest process issues
NP Chart vs. P Chart
• Use NP Chart when sample sizes are constant • Use P Chart when sample sizes vary • NP charts display counts; P charts display proportions
Exam Tips: Answering Questions on NP Chart
1. Identify the Chart Type: When a question mentions counting defective units with a fixed sample size, think NP chart. If sample sizes vary, consider the P chart instead.
2. Memorize the Formulas: Know how to calculate NP-bar, UCL, and LCL. Practice calculating these by hand to build confidence.
3. Watch for Negative LCL: Remember that if your LCL calculation results in a negative number, the LCL should be set to zero. This is a common exam question trap.
4. Understand When to Use NP Charts: Key indicators include: attribute data, constant sample size, counting defective units (not defects per unit).
5. Differentiate from C and U Charts: NP and P charts count defective units. C and U charts count defects (a single unit can have multiple defects).
6. Recognize Out-of-Control Signals: Points beyond control limits, seven consecutive points on one side of the center line, or trending patterns all indicate an out-of-control process.
7. Read Questions Carefully: Pay attention to whether the question asks for the number of defectives or the proportion. This determines whether you use NP or P chart calculations.
8. Practice Sample Problems: Work through multiple calculation problems to ensure you can apply formulas under exam time pressure.