Glossary
PDCA
Plan-Do-Check-Act, or the Deming cycle
The four-step improvement loop - plan a change, run it, measure what happened, then standardise or start again - that underpins most quality management systems.
PDCA comes from statistical process control and was popularised by W. Edwards Deming, who credited it to Walter Shewhart. Plan a change against a stated problem, do it at small scale, check the result against what you predicted, then act: adopt it as the new standard, adapt it, or abandon it.
ISO 9001:2015 is explicitly structured on the cycle, which is why the clause order is not arbitrary. Planning sits in clause 6, operation in 8, evaluation in 9, improvement in 10.
The step that gets skipped is Check. An operation that changes a procedure, never measures whether the defect rate moved, and moves on to the next change is not improving; it is accumulating undocumented variation. The discipline of PDCA is mostly the discipline of finishing the loop.
In a repair workshop the loop is short and concrete: a technician reports that a model keeps failing the same test, a procedure is changed, the next batch is measured, and either the new procedure becomes the reference or it does not. What makes it work is that the loop closes where the work happens, not in a monthly meeting.