In the world of continuous improvement, it might seem that one does not want to look back. After all, as systems improve, old data is no longer useful, and keeping it around—like keeping old love letters—may someday get you into trouble.

Knowing when to recalculate control limits is important, as quality managers know. The traditional seven points above or seven points below the mean, for example, are among several clear signals that can identify the need to investigate and perhaps to recalculate control limits. Recalculating control limits represents an opportunity to move forward, recognizing the dynamic nature of systems and the effects of careful improvement strategies.

New control limits, of course, must be applied only to a current process, thus rendering old information obsolete. There are times, however, when it may be fruitful—or at least interesting—to revisit earlier control limit calculations.

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