SPC Charts (aka “Process Behavior Charts” as Prof. Don Wheeler calls them) are incredibly useful in helping us understand cause-and-effect relationships between attempts at improvement and our key measures.

If you make a change to your system and the next data point(s) are within the range of “common cause variation,” then you might not really have evidence of meaningful improvement. You’d want to see a “signal” in the SPC Chart — something that indicates that the metric is no longer fluctuating around the baseline average level / range of performance.

Here is a related article that I wrote for LEI: https://www.lean.org/the-lean-post/articles/react-less-and-improve-more-by-using-spc-more-effectively/