Management by Criticalities (MBC II)

Concept: Vocabulary that names a phenomenon.

Dan Trietsch’s proposed reformulation of constraint management for stochastic systems where several resources can become economically critical at different times.

Also known as: MBC II; Management by Criticalities; System-Wide Management by Criticalities.

Management by Criticalities is not official TOC doctrine. It is an academic critique of one familiar TOC habit: treating the system as if one constraint can remain the planning center while the rest of the system safely carries protective capacity. Trietsch accepts much of the short-run focusing logic, then asks a harder planning question. When demand, variability, and resource economics change over time, should management keep chasing one named constraint, or should it balance the economic criticality of several resources?

Understand This First

• Five Focusing Steps — the TOC loop Trietsch accepts in part and criticizes in part.

• Drum-Buffer-Rope — the production pattern whose deliberate imbalance is at stake.

• Protective Capacity — the TOC reserve MBC II reframes as a criticality-balancing problem.

• Throughput Accounting — the economic decision frame readers need before weighing the critique.

Context

The standard TOC position starts from focus. Identify the constraint, exploit it, subordinate the rest of the system, elevate when exploitation and subordination are not enough, then return to Step 1 when the constraint moves. In production, that logic supports Drum-Buffer-Rope, Simplified Drum-Buffer-Rope, time buffers, protective capacity, and deliberate imbalance.

Trietsch’s 2005 article, “From Management by Constraints (MBC) to Management by Criticalities (MBC II),” enters from a different doorway. He reads TOC as useful but incomplete for stochastic resource planning. In his framing, short-term constraint focus can still make sense. Medium- and long-term capacity planning needs another rule because resource criticality changes with variability, cost, value, and load. A resource that is not today’s constraint may still be economically critical enough that ignoring it damages future throughput or reliability.

The concept belongs near the TOC field’s live structural debates. Schragenheim and Updegrove’s Fundamental Structure of ToC reorganizes TOC from inside the canon. MBC II presses from outside it. Both matter because they force the same question: is TOC best taught as a historical sequence of tools, or as a set of system principles that must survive criticism from operations research?

What It Is

Management by Criticalities is Trietsch’s proposal to replace long-run “management by constraints” with a resource-criticality frame. The phrase criticality refers to a resource’s economic importance under uncertainty: how much damage the system suffers when that resource is under-protected, overloaded, or badly timed relative to other resources.

In the MBC II argument, constraint focus is too narrow when it is used as a long-term capacity doctrine. A single constraint may bind this week, but the system’s future performance depends on how several resources interact under demand variation, processing-time variation, setup behavior, and capacity cost. The long-run planning problem is therefore not only “which resource constrains now?” It is “which resources are critical enough that capacity, buffers, or policy must be adjusted before the next load pattern makes them binding?”

That is a genuine challenge to TOC’s slogan-level contrast between balanced and unbalanced systems. TOC does not ask for balanced capacity in the Lean takt-time sense. It deliberately keeps protective capacity at non-constraints so the constraint can be exploited. Trietsch argues that the economically correct state may still require balancing criticalities: not equal utilization, but a coordinated level of risk and value across resources.

The result is not a replacement for the Five Focusing Steps in daily execution. It is a competing frame for medium- and long-term planning, especially where the system is stochastic enough that the “one constraint” picture becomes unstable or economically misleading.

How It Is Measured or Recognized

An MBC II situation is recognizable when the current constraint is not enough to explain the system’s economic risk.

One signal is repeated movement among near-constraints. The official TOC answer is often that bad measures, premature release, or exhausted protective capacity are creating wandering bottlenecks. Blackstone’s TOC Handbook literature review makes that case and notes that true product-mix-driven wandering constraints are less common than critics assume. MBC II asks what follows when the movement is real enough to plan around, not merely a symptom of bad local measures.

Another signal is a planning horizon longer than the buffer-management horizon. Daily execution can run on red-yellow-green buffer priorities. Capacity investment cannot. If adding one machine, hiring one scarce skill, or extending one supplier contract changes the criticality of three other resources, the decision needs a multi-resource economic model. MBC II gives that family of decisions a name.

A third signal is a disagreement between local protective-capacity reasoning and system economics. TOC asks whether a non-constraint has productive, protective, or excess capacity. MBC II asks whether the cost of holding that reserve is economically balanced against the risk it protects. The two questions are close, but they are not the same question. Protective capacity can be necessary and still badly placed.

Why It Matters

MBC II matters because it is a named, peer-reviewed objection a serious TOC reader may encounter when leaving the practitioner canon for operations-management literature. The objection should not be waved away as ignorance of TOC. It presses on a real boundary: the difference between short-run operating focus and long-run capacity planning under uncertainty.

The concept also keeps “balance flow, not capacity” from becoming a slogan that answers more than it can. Goldratt’s balanced-plant critique remains central. A plant balanced to local utilization will be fragile under variation. Non-constraints need protective capacity; the rope must prevent premature release; buffers must protect the few places where lost time harms throughput. That does not mean every long-run capacity question is answered by holding one constraint and treating all other resource questions as subordination. Some decisions are about the economic placement of reserve across several possible future constraints.

For certification-track readers, the boundary is practical. The official vocabulary remains Five Focusing Steps, DBR, S-DBR, planned load, buffer management, protective capacity, and Throughput Accounting. MBC II is not a TOCICO exam term in the way those are. But it is part of the scholarly criticism surrounding DBR and capacity planning. Knowing the distinction lets a practitioner say, with precision, “This is an external reformulation, not the certification doctrine, and here is the problem it is trying to solve.”

Common Misunderstanding

“MBC II proves TOC is wrong.” It doesn’t. Trietsch’s argument depends on accepting that focus has value. The criticism is about scope: short-run execution and long-run stochastic planning are different problems. A valid critique of one interpretation of TOC is not a refutation of the whole body of knowledge.

“Criticality means load.” It doesn’t. A resource can be heavily loaded without being economically critical, and a lightly loaded resource can become critical when its variability, setup structure, or capacity cost threatens a high-value commitment. Criticality is about system consequence, not busy time.

“MBC II is just Lean balancing in another vocabulary.” That is too quick. Lean balancing often aims at takt-relative capacity and local waste removal. MBC II is an economic-balancing argument under uncertainty. It may disagree with TOC’s deliberate imbalance, but it doesn’t simply restore local efficiency as the goal.

“TOC has no answer to the critique.” It has several. Protective capacity distinguishes useful reserve from waste. S-DBR planned load watches the CCR without scheduling it in detail. Throughput Accounting and Throughput Economics test larger decisions against T, I, OE, critical resources, and demand uncertainty. The unresolved question is whether those answers are enough in every stochastic, multi-resource planning case. MBC II says no.

“Practitioners should replace the Five Focusing Steps with MBC II.” They shouldn’t. MBC II is a critique and planning frame, not the field’s shared operating language. Use it to understand the debate, especially when academic papers or simulation studies question one-constraint planning. Don’t smuggle it into a TOC implementation as if it were the next official step.

Complements: Fundamental Structure of ToC — Both are structural reformulations of TOC, but Schragenheim and Updegrove work from inside the TOC body of knowledge while Trietsch writes as an academic critic.

Contrasts with: Simplified Drum-Buffer-Rope (S-DBR) — S-DBR monitors planned load on the CCR while keeping the market as strategic constraint; MBC II pushes further toward multi-resource economic balancing.

Depends on: Drum-Buffer-Rope (DBR) — DBR deliberately unbalances the plant around the constraint; MBC II asks when economically balanced resource criticalities are the better planning frame.

Depends on: Five Focusing Steps — MBC II accepts the focusing logic in the short run but challenges the long-run reading of elevation and constraint movement; the reader needs the official loop first.

Depends on: Protective Capacity — Protective capacity is TOC's answer to variation at non-constraints; MBC II reframes the same variation problem as a resource-criticality balance.

Depends on: Throughput Accounting — MBC II's economic-balancing claim sits near the same decision boundary as Throughput Accounting and throughput-per-constraint-unit analysis.

Sources

• Dan Trietsch, “From Management by Constraints (MBC) to Management by Criticalities (MBC II),” Human Systems Management 24 (2005): 105-115. Peer-reviewed source for the MBC II critique and the move from one-constraint focus toward criticality balancing.

• John H. Blackstone Jr., “DBR, Buffer Management, and VATI Flow Classification: A Review of the Literature,” in Cox and Schleier, Theory of Constraints Handbook (2010), Ch. 7 — places Trietsch’s 2005 article inside the DBR research literature and summarizes the related debate over wandering bottlenecks, multiple CCRs, protective capacity, and buffer sizing.

• Shri Srikanth, “DBR, Buffer Management, and VATI Flow Classification,” in Cox and Schleier, Theory of Constraints Handbook (2010), Ch. 8 — official Handbook treatment of DBR, deliberate imbalance, time buffers, the rope, and the protective-capacity logic MBC II challenges.

• Eli Schragenheim, “From DBR to Simplified-DBR for Make-to-Order,” in Cox and Schleier, Theory of Constraints Handbook (2010), Ch. 9 — source for S-DBR, planned load, the market-as-strategic-constraint position, and the distinction between monitoring a CCR and scheduling it as the drum.

• AGI-Goldratt Institute, “Combining Lean, Six Sigma, and the Theory of Constraints to Achieve Breakthrough Performance,” in Cox and Schleier, Theory of Constraints Handbook (2010), Ch. 36 — source for the TOC answer to the balanced-line conflict: protect throughput with deliberate imbalance and focus Lean or Six Sigma work where the constraint makes it matter.

• Goldratt and Cox, The Goal (1984; revised editions 1986, 1992) — originating narrative source for dependent events, statistical fluctuations, the balanced-plant failure, and the “balance flow, not capacity” doctrine that later capacity-planning critiques respond to.