The core distinction

Repair adjusts an element.
Replacement changes the approach.

The 20-minute workflow from A1 identifies what to change. This article covers a prior decision that must be made first: how to change it. The distinction between repair and replacement is not primarily about effort — it is about diagnosis.

Repair is the response to a calibration failure. The right approach was taken but the execution missed the target: the explanation was correct but too abstract, the activity was appropriate but ran too long, the assessment was well-designed but asked the wrong question at the wrong moment. The core approach is sound — it just needs adjustment.

Replacement is the response to a structural failure. The approach itself was wrong: the explanation required prior knowledge students didn't have, the activity produced the wrong cognitive process by design, the sequence made the concept harder to understand rather than easier. No amount of calibration fixes a structural failure, because the problem is in the design, not the execution.

💡The clearest way to hold the distinction
Repairing a structurally broken lesson is like adjusting the font size on a sign that's pointing the wrong direction. The small fix is real work that produces no improvement.
How to tell the difference

The 2-minute diagnosis:
three questions.

The repair-or-replace decision takes 2 minutes when you apply three diagnostic questions in sequence. Each question has a binary output. The sequence of answers routes you to the correct response.

1
Did the same explanation with different wording, a better example, or more time produce correct understanding — even in one or two students?
Yes → leaning repair · No → leaning replace

If a version of the explanation worked — even partially, even for a subset of the class — then the approach itself is viable. The issue is calibration: you used the wrong entry point, the wrong context, or moved too fast. A repaired version of the same approach, better calibrated, will produce the understanding you were targeting.

Signal to watch for
Students who asked questions that showed partial understanding: 'So does that mean X?' — where X is almost right. These students got close via the original approach. A repaired version can close the gap.
2
Did students produce the same wrong answer for a reason that the original approach structurally encouraged?
Yes → replace · No → repair

Some approaches contain the seed of their own failure. An explanation of osmosis using arrows pointing from high to low concentration structurally invites students to attribute agency to the direction — they will say the concentrated side 'pulls'. The explanation created the misconception it was trying to prevent. Repair cannot fix this. A replacement that removes directionality is necessary.

The test
Remove yourself from the lesson and imagine a student encountering this explanation with no prior knowledge. Does the explanation's structure make the wrong answer more plausible than the right one? If yes, the approach is structurally broken.
3
Was the failure because of a missing prerequisite — something students needed to know before this lesson that they didn't?
Yes → repair the sequence, not the lesson · No → continue with repair/replace

A third failure type that is neither repair nor replacement: the lesson itself was sound, but it was delivered in the wrong order. Students needed to understand X before Y made sense, and X hadn't been taught. Repairing or replacing the Y lesson doesn't fix this — addressing X does.

How to identify it
Students can't engage with the activity at all, not just produce wrong answers. Questions reveal no mental model of the relevant concept — not a wrong model, but the complete absence of any model. The lesson needed a different lesson before it.
What each response looks like

Repair: adjust the element.
Replace: swap the approach.

Situation
Repair
Replace
Explanation produced systematic wrong answer
Add a counter-example that directly addresses the wrong answer. Keep the original explanation's framing.
Build a new explanation from a different entry point. Avoid the framing that created the misconception.
Activity produced the wrong cognitive level
Add a constraint that forces the right cognitive level (e.g. "you cannot use an example from today's lesson").
Replace the activity type — if it generated recall, substitute an activity structure that requires application.
Transition lost the class
Add a brief re-anchor at the transition point: a question that bridges the two parts.
Restructure the sequence — the transition failed because the two parts weren't connected at the design level.
Timing collapsed — ran 15 minutes over
Cut one component (identify which is least critical) or add a checkpoint that allows early exit.
Redesign the lesson into two sessions. The content doesn't fit one lesson period — this is a scope problem.
Students tuned out in second half
Add an active task in the second half — the pacing was too passive for too long.
Redesign the second half's purpose. If students tuned out, the second half wasn't generating the right cognitive engagement.
The most common mistake

Replacing when you should
have repaired.

Replacement feels more productive than repair. Rewriting an explanation from scratch feels like real work. Adjusting an existing explanation by adding one counter-example feels minor. This psychological bias — toward action that is visible over action that is targeted — is the most common mistake in lesson iteration.

The cost of unnecessary replacement is high: it takes significantly more time, it loses the institutional knowledge embedded in the original lesson, and it introduces new failure modes. A repaired lesson has a narrow, testable change that will either fix the problem or generate a clear next signal. A replaced lesson has many new variables and makes it hard to tell whether the original problem was solved or simply obscured.

⚠️The replacement heuristic
Before replacing any element, ask: can I produce the correction I need in under 5 minutes? If yes, repair is the right call. If fixing the element requires more than 5 minutes of writing, that is likely because you are replacing it while telling yourself you are repairing it. Stop. Check: is the approach itself broken, or just badly calibrated? The answer determines the 20-minute allocation.