What type of chart is used to monitor the proportion defective in a sample?

• A. X-bar chart monitors the proportion defective in a sample.
• B. P-chart monitors the proportion defective in a sample.
• C. S chart monitors the sample standard deviation.
• D. C-chart monitors the number of defects per unit.

Answer: (B)

When you’re evaluating whether items are defective or not, you’re working with binary (attribute) data. The chart designed for this is the P-chart, which tracks the proportion defective in each sample. You compute p-hat for each sample as the number defective divided by the sample size, then plot those proportions with a center line at the average defect rate p̄ and control limits based on p̄ and the sample size n (p̄ ± z√[p̄(1−p̄)/n]). If sample sizes vary, the limits adjust with the specific n for each sample.

Other charts serve different data types: X-bar and S charts monitor the mean and variability of a measured quantity (variable data), C-chart counts the number of defects per unit, and U-chart tracks defects per unit when unit sizes vary. So for monitoring the proportion defective in a sample, the P-chart is the appropriate tool.