What is the purpose of a control chart in maintenance metrics?

• A. Replaces root cause analysis.
• B. Tracks performance over time to identify natural vs assignable variation.
• C. Predicts exact failure times.
• D. Measures only daily downtime.

Answer: (B)

The key idea behind a control chart in maintenance metrics is to monitor a metric over time and distinguish natural, everyday variation from unusual, assignable variation. By plotting the metric (such as downtime, MTBF, or repair time) with a center line (the average) and upper/lower control limits, you can see whether fluctuations are just normal noise or signaling a change in the process. When data stay within the limits and look random, the process is in statistical control and no special investigation is needed. If you observe points outside the limits or nonrandom patterns (like a trend or consecutive points on one side), that indicates an assignable cause likely needing investigation and corrective action. This approach doesn’t predict exact failure times, nor does it replace root cause analysis; it serves to alert you to when to look deeper. It also isn’t limited to daily downtime—any maintenance metric tracked over time can be monitored this way.