Which chart should you use to monitor defects when the sample size changes?

The u-Chart is the appropriate choice for monitoring defects when the sample size varies. This type of control chart is specifically designed for cases where the individual inspection units can differ in size or number, allowing you to track the average number of defects per unit.

In a u-Chart, the focus is on the average number of defects per unit, making it essential for analyzing situations with variable sample sizes. This flexibility enables quality analysts to accurately assess the process performance and control when working with data that do not consistently reflect the same volume of output or inspection units.

In contrast, other charts like the c-Chart are used when the sample size remains constant but focus solely on the count of defects in a fixed sample, making them unsuitable for situations with varying sample sizes. Similarly, np-Charts and P-Charts have specific applications related to the number of defectives in fixed size samples or proportions of defectives, which do not apply when sample sizes are changing. Thus, the u-Chart effectively accommodates fluctuations in the number of units inspected, providing a more accurate representation of defect trends in such scenarios.