Which control chart is used to monitor the number of defects when the opportunity count varies?

Question

Options

• AC-chart
• BP-chart
• CNP-chart
• DU-chart

• A
  4% (2)
• B
  11% (6)
• C
  6% (3)
• D
  80% (43)

Explanation

U-chart (D) is correct because it monitors the number of defects per unit when the opportunity for defects varies across samples - for example, inspecting fabric bolts of different lengths or circuit boards with different component counts. By expressing defects as a rate per unit, the U-chart normalizes for varying sample sizes, making comparisons valid.

Why the distractors are wrong:

• C-chart (A) also tracks defect counts, but requires a constant opportunity size - use it when every sample has the same inspection area.
• P-chart (B) handles variable sample sizes, but monitors the proportion of defective items (pass/fail), not the count of defects within items.
• NP-chart (C) monitors the number of defective items, but requires a constant sample size.

Memory tip: Think of the four charts in a 2×2 grid - rows are "defects" vs "defectives," columns are "constant" vs "variable" sample size. U = defects + variable (U for "Unequal units"). If you can remember that U-chart is the defect-counting chart that handles unequal sample sizes, the others fall into place by elimination.

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