It is a basic of formal problem solving: make sure the solution solves the problem for good.
The PDCA cycle reminds that problem solving does not end with the first best solution put in place, but the results have to be assessed (C for Check) and a next step envisioned (A for Act), closing a loop too often left open.
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Yet monitoring results of a solution during a limited time may not suffice to state the problem is solved. Some nasty problems show up with (very) low frequency.
In order to check the disappearance of the problem in the long term, the Quality Operating System (sometimes called Business Operating System) combines a Pareto chart with a histogram on a wide time span, usually at least twelve months rolling.
The Pareto chart on the left side is used as usually, to focus on the most important actual issues.
The histogram on the right side monitors the occurrences of problems for which solutions were put on place over time.
If the occurrences decrease or disappear steadily, problem can be assumed solved after x (12 or more) months.
If the problems reappears, the solution obviously didn’t kill the root cause.
In the example hereunder each quality issue is given a distinct color in order to spot one easily in the charts.
See the Black one.
It had been a big trouble 16 months ago, but thanks to the proposed solution, the occurrences decreased steadily in time (couldn’t be instant because of WIP and inventories to flush). There was no surging of this problem again and sixteen months later we have less than 3 occurrences.
The Orange issue is more or less the same story than Black.
The Blue problem is still not solved after 16 months, as the histogram shows and is the number two concern this month. The solutions tested so far obviously weren’t good.
The Red problem is four months old and still a great pain that wasn’t addressed efficiently until now.