Which chart is best suited for measuring 'defects' when the sample size may increase or decrease?

The u-Chart is particularly well-suited for measuring 'defects' when the sample size may vary because it is designed to monitor the proportion of defects per unit in a given sample. Unlike other charts that may be applicable in scenarios with a fixed sample size, the u-Chart allows for flexibility in the number of units observed, making it ideal for situations where the total number of observations can change from one sample to another.

This chart calculates the average number of defects per unit, which enables analysts to assess process stability and performance under different sampling conditions. It provides insights into the process's capability to maintain quality over time, even as the amount of data collected fluctuates, thus offering a more nuanced understanding of defect rates relative to the sample sizes taken.

In contrast, other charts like the P-Chart and np-Chart are more appropriate when dealing with a consistent sample size, with the P-Chart focusing on proportions of defectives rather than the average defects per unit. The c-Chart is used exclusively for counting defects in a constant sample size, further highlighting the u-Chart's unique adaptability to changing sample sizes for measuring defects effectively.