GB grid frequency

Tick a rule off and its points, its shaded runs and its card below the chart go with it — nothing is refetched, so you are looking at the same readings judged by fewer rules. Change how often a reading is taken and watch the limits move. Sample too often and every reading is almost a copy of the one before it — the chart then thinks the process is far steadier than it is, and draws limits so tight that normal variation looks like a crisis. Only the setting marked (the real one) shows the limits that were locked at baseline and have never been recalculated since.