Under what conditions would you use an individuals (I) chart instead of an X-bar chart?

• A. When data are in large subgroups with many observations per subgroup.
• B. When you have multiple measurements per subgroup only.
• C. When you want to compare two processes.
• D. When you have single, non-subgrouped measurements or when subgroups are not possible.

Answer: (D)

When you only have single measurements or cannot form subgroups, you use an individuals chart. It plots each individual value and, to gauge dispersion, typically uses a moving-range chart alongside it. X-bar charts, by contrast, monitor the average of subgroups and require multiple observations per subgroup to calculate those means. If you don’t have multiple observations in each subgroup or can’t create subgroups at all, an individuals chart is the appropriate tool. That’s why the scenario described—single, non-subgrouped measurements or subgroups not possible—best fits using an individuals chart.