Which type of control chart would you use for counting defectives when the sample size varies?

The P-Chart is the appropriate choice when you need to monitor the proportion of defectives in a sample when the sample size can vary. This type of control chart is specifically designed to handle data that represent proportions, making it suitable for situations where you're interested in the fraction of items that are defective across different sample sizes.

In a P-Chart, each point on the chart represents the proportion of defectives found in a sample, allowing you to visualize how this fraction fluctuates over time. The chart also accommodates variations in sample size by calculating the control limits based on the size of each individual sample, ensuring that the analysis remains valid even when samples differ in size.

Other chart types such as the np-Chart, c-Chart, and u-Chart serve different purposes. The np-Chart is used only when the sample size remains constant, while the c-Chart and u-Chart are suitable for counting the number of defects rather than the proportion of defectives and are used for fixed counts of units or when the sample size is uniform. Thus, the P-Chart is distinctly suited for the scenario where the sample size varies, making it the most effective tool for monitoring defectives over time in such cases.