When you work within a continuous improvement culture, it might seem that one does not want to look back. After all, as systems improve, old data may no longer be relevant for the current process, and keeping it around—like keeping old love letters—may someday get you into trouble.

Knowing when to calculate new control limits is important, as quality managers know. The traditional rule of seven-points-above or seven-points-below the mean can identify the need to investigate and perhaps to calculate new control limits. Calculating 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 the current process and how it is behaving, thus rendering old information obsolete. There are times, however, when it may be fruitful, or at least interesting, to revisit earlier control limits. Reviewing previous control limits may shed light on how a system has changed, or provide insight about patterns that recur in a system.

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