np chart
np chart is a type of control chart used in statistical process control to monitor the number of defective items in a process over time with constant sample sizes.
Map the data fields
- Subgroup/Lot: This field represents the different lots or batches of items being inspected.
- Process variable: This field contains the number of defective items in each lot.
- Sample Size: This field indicates the number of items inspected in each lot (should be constant for np charts).
- UCL: Optional field for custom upper control limits.
- LCL: Optional field for custom lower control limits.
- Central Line: Optional field for custom center line values.
Calculation Methodology
The np-chart calculations are performed as follows:
Proportion defective (p̄)
p̄ = Total Defectives / Total Samples
Number of defectives (np̄)
np̄ = Sample Size × p̄
Standard Deviation
Standard Deviation = √(np̄ × (1 - p̄))
Control Limits
For each data point:
Upper Control Limit (UCL):
UCL = np̄ + (Z-Score × Standard Deviation)
Lower Control Limit (LCL):
LCL = np̄ - (Z-Score × Standard Deviation)
Center Line (CL):
CL = np̄ (expected number of defectives)
Actual Defectives for Each Sample:
Number of Defectives = Defectives count for each lot
Notes:
- If custom limits are provided, they override the calculated values
- The Z-Score determines the confidence level (typically 3 for 99.7% confidence)
- np charts are used when sample sizes are constant across all lots
- The actual number of defectives is plotted directly (not as a proportion)
- Control limits are calculated based on the expected number of defectives per sample