What is a control chart primarily used for in quality management?

A control chart is primarily used to study process changes over time. It provides a visual representation of process data, allowing management and quality control teams to monitor variation in a process over specific periods. By plotting data points over time, it becomes easy to identify trends, shifts, or any unusual patterns that may suggest that a process is going out of control due to variations that are either common or assignable causes. This helps organizations maintain quality and consistency in their outputs by making timely decisions based on observed data.

While the other options pertain to quality management processes, they do not capture the primary function of a control chart. For instance, comparing variables may involve different statistical methods, and ranking causes of problems typically uses techniques like root cause analysis rather than control charts. Similarly, collecting qualitative data is a different approach that does not focus on quantitative process monitoring, which is the main strength of a control chart.