PDCA, PDSA, SDCA: a wheel of confusion if we are not careful.
To PDCA or to PDSA?
For decades, organisations around the world have been encouraged to adopt a simple, scientific cycle for improvement: PDCA. Or was it PDSA? And what about SDCA? Hang on a minute, my wheel is spinning.
The confusion is understandable. I’ve heard the terms often used interchangeably, and many training programs attribute PDCA directly to W. Edwards Deming, the American statistician who helped inspire Japan’s post-war quality revolution. And if I’m perfectly truthful, so did I until a bright spark told me to ‘do my research’. So I finally had the time to do some digging.
Deming did not support PDCA
Deming did not teach PDCA in the form most people know today. PDSA (not PDCA) was Deming’s preferred cycle. Furthermore, SDCA is not a Deming concept at all.
For clarification:
PDCA, as we all know, stands for Plan Do Check Act
PDSA, Deming’s own preferred wheel, stands for Plan Do Study Act
SDCA is another manipulation from the Lean/Lean Six Sigma/Six Sigma community; another fudge in my view.
So, to unpack this further, here is what Deming actually said, and an explanation of why Toyota can thrive with PDCA while many Western organisations unintentionally sabotage their improvement efforts.
Where the Confusion Began: Shewhart, Deming, and a Mistranslation
So this is how we think the story goes:
Shewhart created the improvement cycle. The more I learn about this cheeky chappy’s work, the more in awe I am.
Deming sauntered over to Japan and taught the cycle. This we all know.
Japan popularised PDCA.
PDCA spread to the West.
Simple… Except there is a catch.
1. Shewhart’s original cycle was not PDCA. Oh no, here we go.
Walter A. Shewhart, Deming’s mentor at Bell Labs, introduced a scientific cycle involving specification, production, and then inspection. It was rooted in experimentation and learning, but did not use the PDCA terminology.
The PDCA framing was a later Japanese interpretation meant to simplify and teach the Shewhart Cycle. Yes, that’s right, it’s a Japanese flip on Shewhart’s cycle prior to Deming’s arrival in Japan! In his book, Out of the Crisis (1986), pp. 88–94, Deming describes Shewhart’s cycle and explicitly states that PDCA is a Japanese adaptation.
Deming rejected Check and replaced it with Study.
Deming believed the word Check was dangerously misleading. Why? Because in English, Check is tied to inspection, audit, verification, and policing; all activities that take place after an outcome is produced. For Deming, true improvement requires learning, not judging. So he replaced Check with Study.
“I don’t use check. Check is too closely associated with inspection.” The New Economics (1993), Deming states that PDCA is wrong and that the correct cycle is PDSA. Oh boy. For Deming, his concern was that the scientific method would be lost with the word check, and that we would revert to our old ways of inspecting and auditing quality within the process.
He wanted experimentation in the classic sense, and quite right too!
Plan: Form a hypothesis and make a prediction.
Do: Conduct a controlled test.
Study: Compare results to predictions; understand variation.
Act: Adopt, adapt, or abandon.
Deming’s philosophy was grounded in learning about systems, not policing results. PDSA reflects this intent.
SDCA is not a Deming concept
SDCA: Standardise-Do-Check-Act, emerged later in Lean and Six Sigma circles. It is often framed as a cycle for maintaining standardised work, or in some cases, before attempting improvement, one must standardise.
Deming absolutely valued standardisation as a basis for learning, but he never used SDCA. As good consultants, we have just added a juicy twist to show how smart we all are, and in the process, confused the hell out of everyone.
So, Why Does Toyota Use PDCA, and Why Does It Work So Well?
Here’s where things get interesting.
Toyota (the global benchmark for Lean and continuous improvement) uses PDCA everywhere. It is part of daily management and is applied from frontline kaizen to executive decision-making.
If Deming preferred PDSA, how can Toyota rely on PDCA without falling into the ‘check = inspection’ trap? My punt is that it’s Toyota’s culture that transforms what PDCA means.
Toyota’s Cultural Advantage: Why PDCA Behaves Like PDSA Inside Toyota
If I dare to be so bold, in Toyota, Check means Study in practice
Toyota’s interpretation of Check aligns closely with Deming’s meaning of Study.
Inside Toyota:
1 Toyota’s foundational principles: genchi genbutsu (go see), hansei (reflection), kaizen (continuous improvement)—ensure that Check is a learning activity.
Checking is reflection, not inspection. It invites curiosity rather than compliance.
In other words: Toyota can safely use PDCA because they live PDSA.
2 Toyota exhibits ‘healthy dissatisfaction’, where standards are never final. Toyota is never satisfied with the status quo.
At Toyota:
- Standards are temporary best-known methods, not permanent solutions.
- Improvement is part of the job, not a special project.
- Leaders expect teams to challenge the standard daily.
- Experimentation is normal and supported.
- They intentionally cultivate a sense of constructive restlessness.
And the result?
The Act step, often skipped in other organisations, is where Toyota excels. They adopt, refine, or abandon with discipline and regularity.
And so back to Deming:
“There is no true resting place in improvement.” Deming, Out of the Crisis. Toyota internalised this at a cultural level.
3. Western organisations often lack these cultural foundations
In contrast, many Western companies unintentionally reinforce the exact pitfalls Deming warned about:
- Standards are seen as things to lock down, not starting points for learning.
- Improvement is episodic (projects, events), not daily work.
- Leaders manage results instead of coaching learning.
- Spans of control are too wide for meaningful mentorship.
- Check becomes an audit, inspection, or performance evaluation.
- Variation is blamed on individuals rather than understood as systemic.
This is why PDCA often degenerates into simple compliance loops in the West.
In these environments, PDSA may be a better way of forcing leaders to think scientifically rather than judgmentally.
When we step back, one insight becomes crystal clear:
The improvement cycle works when the culture supports learning. It fails when the culture defaults to compliance. Terminology matters far less than the behavioural norms behind it.
Conclusion: Deming’s Lesson Still Matters, But Culture Is the Missing Ingredient
The debates over PDCA, PDSA, and SDCA often focus on acronyms.
But the truth is simpler:
Deming taught PDSA to emphasise learning.
Toyota uses PDCA because its culture naturally supports learning.
Western companies often misinterpret PDCA because their cultures default to inspection and compliance. If we want continuous improvement to flourish, organisations must accept that the real work is in leadership behaviours, psychological safety, reflection habits, and systems thinking.
Thank you, bright spark
