Defective Count SPC for Fixed Subgroups
Operating directly on defect counts eliminates rounding discrepancies and simplifies communication between production operators, line leaders, and quality assurance personnel.
Foundational Category: Statistical Process Control (SPC) • ~5 min read
np Chart for Number of Defectives
Simplify production floor quality tracking with integer-based defective unit counts.
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Q:
When should an np-Chart be chosen over a p-Chart?
Choose an np-chart when inspection subgroup size n is strictly constant across all sampling intervals. Instead of tracking fractional percentages, it tracks integer counts of defective units, making it intuitive for machine operators on the shop floor.
CL = n×p̄, UCL = n×p̄ + 3√(n×p̄(1-p̄)), LCL = Max(0, n×p̄ - 3√(n×p̄(1-p̄)))
Centerline (np̄)
5.87
Avg defectives / lotUpper Limit (UCL)
13.03
LCL: 0.00Fraction Defective (p̄)
2.93%
Rate per unitOut of Control
0 Subgroups
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These tools and calculators are intended for engineering analysis, educational study, and operational guidance. They do not constitute formal regulatory certification (such as medical, aerospace flight-critical, or statutory safety filings) without independent audit and sign-off by a certified Quality Engineer or Six Sigma Black Belt. AshToolbox is not affiliated with, sponsored by, or endorsed by AIAG, ISO, ASQ, or any standards body.
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