What type of control chart should be used to study the above data? Answer: p chart with constant subgroup size

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• Ap chart with constant subgroup size

• A
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Explanation

A p chart monitors the proportion (fraction) of defective items in a subgroup, making it ideal when data is attribute-based (pass/fail, conforming/nonconforming) and sample sizes remain equal across subgroups - exactly the scenario described. With constant subgroup sizes, the control limits are uniform across all points, simplifying interpretation and calculation.

Why distractors are wrong (common alternatives):

• An np chart also requires constant subgroup sizes but tracks the count of defectives, not the proportion - use it only when reporting raw counts is more meaningful than a rate.
• A c chart tracks the count of defects (not defectives) per inspection unit with a fixed area of opportunity - a different attribute type entirely.
• A u chart handles defects per unit when the subgroup (opportunity) size varies - the opposite of this scenario.
• A p chart with variable subgroup size is only needed when sample sizes differ between subgroups, which recalculates control limits for each point.

Memory tip: Think "p = proportion, constant = uniform limits." If you see attribute data (defective/not defective) with the same sample size every time, reach for the p chart. The "p" stands for proportion, and equal subgroup sizes keep everything neat and constant.

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