Preload, prefetch and fetch priority all influence resource fetching, but they express different intentions. Preload discovers a resource needed by the current page early. Prefetch hints that a resource may help a future navigation. fetchpriority adjusts the relative priority hint for a fetch the browser already understands.
This guide develops one focused reader question within the Websites subject guide. It adds a complete mechanism, original worked decisions, changed-context transfer checks and explicit limits. Read it with The Gold Standard of Education | Learning That Transfers into Life and the How X Works.
All numerical examples are invented teaching cases. They are not operational measurements, production records, financial advice, conservation findings or compliance results.
For a fictional page, hero.avif is needed for the current page and discovered late in CSS, while article-next.html may be used only after a link click. Preload hero.avif with the correct as and type attributes; prefetching it treats a current-page need as speculative. Prefetch article-next.html only if the likely navigation and cache rules justify the bytes. A high fetchpriority on the hero can complement preload, but it neither guarantees scheduling nor repairs an incorrect URL or mismatched credentials mode. MDN preload, MDN prefetch and MDN fetchpriority document these distinct semantics and browser-dependent effects.
Continue with Separating Field Performance Data from Laboratory Test Results, Separating Compressed Transfer Size from Decoded Resource Size and the Websites root.
The short answer is to keep the target quantity, observation and reporting rule separate. Name what the system intends to estimate, what was actually observed, how the summary was formed and which conditions make the inference defensible. If one field is missing, keep the ambiguity visible.
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Preload, prefetch and fetch priority all influence resource fetching, but they express different intentions. Preload discovers a resource needed by the current page early. Prefetch hints that a resource may help a future navigation. fetchpriority adjusts the relative priority hint for a fetch the browser already understands.
ROUTE 4 · CHAPTERS 19–22
Audit and continue
Repair claims, answer questions and follow verified routes.
Chapter contents
Define the question · Chapters 1–6
2. Separate the two target quantities
4. Protect the denominator and boundary
Protect the method · Chapters 7–12
7. Check units before interpretation
8. Keep method settings visible
10. Recognise saturation and censoring
Test the claim · Chapters 13–18
13. Separate mechanism from appearance
16. Change one consequential condition
Audit and continue · Chapters 19–22
19. State what it cannot establish
20. Repair an overconfident sentence
Start with when the resource is needed: current render, later interaction or possible future navigation.
Write an evidence ledger before calculating: target quantity, object, unit, boundary, observation rule, transformation and decision limit. Mark every input as observed, stipulated, fitted or assumed.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “define the reader question”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Preload declares an early current-page fetch; prefetch is speculative future use; fetchpriority is a relative hint.
Draw the mechanism in order. Separate the underlying state, the measurement, the mathematical summary and the claim. Then name one rival explanation and the additional observation that could distinguish it.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “separate the two target quantities”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Record rel, as, type, crossorigin, URL, initiator and cache policy. One wrong field can cause duplicate or unusable work.
Keep the comparison fair by holding the specimen, interval, protocol and denominator constant. If one changes, state that the new number answers a different question even when the label looks familiar.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “name the observation rule”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Measure bytes, request count and critical path separately; a faster hero can coexist with more wasted transfer.
Use two checks that can fail differently: reconstruct a total, test a limiting case, repeat the result from definitions or inspect a shared control. Two calculations that reuse one assumption are only one check.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “protect the denominator and boundary”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
List each candidate resource, discovery time, consumer, current-page need and fallback before adding hints.
Separate measurement from decision. A correct number can still be too local, uncertain or method-dependent for the intended action. Report it with its conditions and say what stronger evidence would be needed.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “build the fictional evidence ledger”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
The hero is current-page critical, while the next article is conditional. Assign preload to the former and cautious prefetch to the latter.
| Resource | Need | Candidate hint |
|---|---|---|
| hero.avif | current page | preload, possibly high priority |
| article-next.html | possible next page | prefetch or speculation rule |
Preserve identity and order. Equal totals can hide different mechanisms when records come from different specimens, resources, homes or viewing geometries. Aggregate only after the question no longer depends on that identity.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “work the central calculation”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Milliseconds, bytes and priority labels are different quantities. Preserve each rather than forming one speed score.
Run a changed-context transfer check. Change one consequential condition while holding the remaining basis fixed, calculate again and explain why the result moves or stays stable.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “check units before interpretation”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Browser version, cache partitioning, network state, Save-Data behavior and server headers affect the outcome.
Put the boundary of inference beside the answer. Name the system, representation and interval to which the result applies, then name the nearest tempting conclusion the evidence does not establish.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “keep method settings visible”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Inspect waterfall timing, request initiators, priority changes and whether the eventual consumer reuses the response.
Use uncertainty as structure. Vary the input most capable of changing the conclusion over a defensible range and show whether the ranking or interpretation changes.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “locate the usable range”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Preloading many assets can compete with CSS, fonts or scripts. Priority hints are scarce scheduling signals.
Finish with a reproducible evidence package: source records, definitions, exclusions, calculations, validation result and revision rule.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “recognise saturation and censoring”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Test cold and warm cache, throttled and unthrottled conditions, and a control without hints.
Write an evidence ledger before calculating: target quantity, object, unit, boundary, observation rule, transformation and decision limit. Mark every input as observed, stipulated, fitted or assumed.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “use controls that can fail differently”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Use repeated traces and field evidence; one lab waterfall cannot establish population-wide benefit.
Draw the mechanism in order. Separate the underlying state, the measurement, the mathematical summary and the claim. Then name one rival explanation and the additional observation that could distinguish it.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “carry uncertainty into the claim”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
A late hero may come from CSS discovery, server delay, decoding or layout. A resource hint addresses only some paths.
Keep the comparison fair by holding the specimen, interval, protocol and denominator constant. If one changes, state that the new number answers a different question even when the label looks familiar.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “separate mechanism from appearance”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Keep page, device, cache and connection conditions aligned when comparing variants.
Use two checks that can fail differently: reconstruct a total, test a limiting case, repeat the result from definitions or inspect a shared control. Two calculations that reuse one assumption are only one check.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “compare like with like”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
If a preload is fetched twice, check URL equivalence, as, type and crossorigin before blaming the browser.
Separate measurement from decision. A correct number can still be too local, uncertain or method-dependent for the intended action. Report it with its conditions and say what stronger evidence would be needed.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “test a rival explanation”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Change the hero to an above-fold CSS background already discoverable early. The value of explicit preload may shrink.
Preserve identity and order. Equal totals can hide different mechanisms when records come from different specimens, resources, homes or viewing geometries. Aggregate only after the question no longer depends on that identity.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “change one consequential condition”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
If median hero start improves 120 ms but total transferred bytes rise 300 kB from speculative prefetches, report both effects.
Run a changed-context transfer check. Change one consequential condition while holding the remaining basis fixed, calculate again and explain why the result moves or stays stable.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “recalculate the changed case”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
The trace can establish earlier discovery and reuse under the tested browser and cache state.
Put the boundary of inference beside the answer. Name the system, representation and interval to which the result applies, then name the nearest tempting conclusion the evidence does not establish.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “state what the result establishes”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
It cannot guarantee user-perceived improvement, future navigation or identical behavior across browsers.
Use uncertainty as structure. Vary the input most capable of changing the conclusion over a defensible range and show whether the ranking or interpretation changes.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “state what it cannot establish”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Replace “fetchpriority makes the image load first” with “it supplies a browser-dependent relative priority hint.”
Finish with a reproducible evidence package: source records, definitions, exclusions, calculations, validation result and revision rule.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “repair an overconfident sentence”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Does preload execute a script? No. It schedules a fetch; normal consumption still matters.
Is prefetch guaranteed? No. It is a hint and may be ignored.
Should every image be high priority? No. Overuse can delay truly critical work.
Write an evidence ledger before calculating: target quantity, object, unit, boundary, observation rule, transformation and decision limit. Mark every input as observed, stipulated, fitted or assumed.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “answer focused reader questions”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.
Use browser documentation and measurements, then follow field-versus-lab and transfer-size branches.
Draw the mechanism in order. Separate the underlying state, the measurement, the mathematical summary and the claim. Then name one rival explanation and the additional observation that could distinguish it.
Apply the rule to the central fictional case with unrounded inputs, then label the result with its object, unit and protocol. Name one alternative explanation and the observation that would distinguish it.
For Websites, complete seven fields: target, object, unit, boundary, protocol, result and limit. For “continue through verified routes”, a plausible number can otherwise hide a changed definition or undocumented condition.
Change one condition that matters while holding the remaining basis fixed. Recalculate or re-inspect the evidence, state what stays comparable and name the stronger conclusion that still does not follow.
Close with two lines. The first states what the evidence establishes under the named conditions. The second states the nearest tempting conclusion that remains unsupported. Ask another reader to identify the condition that would make the first line false.
Evidence check: preserve the raw observation, the transformation, the comparison rule and the failed alternatives in one audit note. A future reader should be able to reproduce the narrow result without guessing which conditions were held constant.