Could the tacit-cost gap (concurrent vs. silent) serve as a measure of expertise?

Whether the tacit-cost gap — the difference between an expert's judgment consistency when silent (full tacit + explicit) and when verbalising concurrently (explicit only) — could serve as a measure of expertise, and whether the gap widens (the tacit layer grows, pulling the expert further from the verbalisable) or narrows (the expert's explicit layer becomes richer and more predictive) as expertise deepens.

The three-condition protocol already produces the tacit-cost gap as a byproduct, and the question is whether this byproduct is also a measure. The three-condition-protocol room designed three conditions — silent (the judgment made without verbalisation), concurrent Type 1 (say what is in working memory, never explain why), and retrospective (narrate after the fact) — and predicted two gaps: the reconstruction cost (concurrent vs. retrospective) and the tacit cost (concurrent vs. silent). The tacit-cost gap is the difference between the expert's judgment consistency when silent and when verbalising concurrently: if the gap is large, verbalising stripped away a layer that silent judgment used; if the gap is small, the verbalisable layer was most of what the expert was using anyway. The question is whether this gap, measured across experts of differing skill, tracks expertise (read 2026-06-21 — three-condition-protocol room (castle, built 2026-06-21)).

ACT-R's declarative-to-procedural transition predicts the gap widens: as expertise deepens, more of the judgment moves into production rules that are not in working memory. ACT-R (Adaptive Control of Thought—Rational) holds that cognitive skill develops in two stages: first, knowledge is encoded as declarative chunks (examples, facts, explicit rules) that are consciously accessed and slowly applied; with practice, these are compiled into production rules — procedural knowledge that fires automatically without occupying working memory. The transition is one-way: declarative becomes procedural, not the reverse. As more of the expert's judgment runs through productions, less passes through working memory, and the concurrent protocol (which captures only working-memory contents) captures less of the judgment. The tacit-cost gap widens because the silent expert uses productions the concurrent verbaliser cannot report (read 2026-06-21 — Wikipedia: ACT-R (read 2026-06-21); Wikipedia: Procedural knowledge (read 2026-06-21)).

But the expertise reversal effect predicts the gap narrows for a different reason: the expert's explicit layer becomes richer, not poorer. The expertise reversal effect found that instructional techniques helping novices build schemas become redundant for experts who already have them — but the experts' schemas are not tacit, they are explicit and well-organised. An expert's declarative knowledge is richer (more connections, more accessible schemas) than a novice's, and the expert can articulate more of their reasoning, not less, because they have more to articulate. On this reading, the tacit-cost gap narrows: the concurrent protocol captures more of the expert's judgment because more of it is explicit (rich schemas, articulated criteria), and the silent advantage shrinks. The expertise reversal effect says the expert's advantage is schema-based, and schemas are declarative, not procedural (read 2026-06-21 — Wikipedia: Expertise reversal effect (read 2026-06-21)).

Polanyi's "we can know more than we can tell" predicts the gap widens, but Ryle's know-how/know-that distinction says the gap measures proceduralization, not expertise per se. Polanyi's tacit dimension holds that all knowledge is rooted in tacit knowledge, and the expert's edge is precisely what they cannot articulate — "we can know more than we can tell." Ryle's distinction says the expert's know-how is a different kind of knowledge from know-that, not a lesser version of it. The tacit-cost gap, on this reading, measures the ratio of know-how to know-that in a given judgment: a wide gap means the judgment runs mostly on know-how (procedural, automatic), a narrow gap means it runs mostly on know-that (declarative, explicit). The gap measures how proceduralized the judgment is, not how expert the judge is — and a judgment can be expert without being proceduralized (a rich declarative schema) or proceduralized without being expert (a well-practiced but shallow routine) (read 2026-06-21 — Wikipedia: Tacit knowledge (read 2026-06-21); Wikipedia: Procedural knowledge (read 2026-06-21)).

The power law of practice and automaticity suggest the gap widens smoothly at first, then the question is whether it jumps or plateaus. The power law of practice holds that reaction time decreases logarithmically with practice trials — a smooth curve, not a step. Automaticity develops gradually: the centipede effect (conscious reflection disrupting automatic performance) appears when attention is drawn to a process that has already become automatic, suggesting there is a threshold where the process leaves working memory. If the tacit-cost gap tracks automaticity, it should widen smoothly with practice (more trials → more proceduralization → wider gap) and then either plateau (once the judgment is fully proceduralized, more practice does not widen it further) or jump (if proceduralization happens at a discrete point). The three-condition-protocol room's uncertain: note asked whether the task is slow enough for concurrent verbalisation; the same uncertainty applies here — if the judgment is fast and automatic, the concurrent condition may be impossible without distortion, and the gap is unmeasurable (read 2026-06-21 — Wikipedia: Power law of practice (read 2026-06-21); Wikipedia: Automaticity (read 2026-06-21)).

The honest state. The tacit-cost gap could serve as a measure of expertise, but what it measures is proceduralization (the ratio of automatic to explicit processing in a judgment), not expertise per se. ACT-R and Polanyi predict the gap widens (more of the expert's judgment moves out of working memory into productions); the expertise reversal effect predicts it narrows (the expert's declarative schemas become richer and more articulable). The two predictions are not contradictory but describe different kinds of expertise: the proceduralized expert (the centipede that walks without thinking) has a wide gap, the schema-rich expert (the scholar who can articulate every move) has a narrow gap. Whether the gap is a continuous measure or jumps at the point of proceduralization is the same smooth-or-thresholded question the threshold-pilot room asked of concept teachability — and the answer is the same: uncertain, buildable, unbuilt. The tacit-cost gap is a candidate measure of one kind of expertise (procedural depth), not of expertise itself, and its validity depends on whether the expertise being measured is the kind that proceduralizes or the kind that enriches the explicit layer.

uncertain: whether the tacit-cost gap can be measured at all for fast automatic judgments — if the expert's judgment takes under a second, concurrent verbalisation may be impossible without slowing the judgment (introducing reactivity), and the gap is unmeasurable rather than zero.