fix: Unweighted Oracle Standard Deviation Inflates Reward Band

[mattkii]

Description

The oracle picks a "median" price weighted by validator stake, but it measured how spread out the votes were without using stake and divided by the number of votes instead of total stake. Because of that mismatch, a few small validators could submit way off prices to make the spread look bigger, which widened the "acceptable" range and let bad votes still count as correct.

The fix weights each vote by its stake when measuring the spread, and divides by total stake so the spread now matches the stake-weighted median.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

go test -tags=test ./x/oracle/...

• Updated the standard-deviation unit test to expect the stake-weighted result
• All oracle tests pass

PR Checklist:

• Updated changelog with PR's intent
• Lint with make lint-fix

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 481dcf60-0724-4de1-81ff-2c8b2fbef99f

📥 Commits

Reviewing files that changed from the base of the PR and between ac6cf8a and e5078ec.

📒 Files selected for processing (1)

• CHANGELOG.md

✅ Files skipped from review due to trivial changes (1)

• CHANGELOG.md

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Walkthrough

The oracle module's StandardDeviation function in ballot.go now computes variance by weighting each vote's squared deviation by its voting power and dividing by total voting power, with a zero-power guard added. The corresponding test expectation was updated, and the changelog records the fix.

Estimated code review effort: 2 (Simple) | ~10 minutes

Changes

File	Summary
x/oracle/types/ballot.go	StandardDeviation now computes power-weighted variance normalized by total power, with an early zero-power return.
x/oracle/types/ballot_test.go	Updated expected standard-deviation value in TestStandardDeviation.
CHANGELOG.md	Added a Fixed entry documenting the stake-weighted variance change.

Sequence Diagram(s)

Not applicable.

Related issues: None specified.

Related PRs: None specified.

Suggested labels: bug, oracle

Suggested reviewers: None specified.

Poem
A rabbit weighed each vote with care,
Power tipped the variance there,
Median stayed, the window tight,
Low-stake votes no longer steer it wide.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main bug fix in oracle standard deviation weighting.
Description check	✅ Passed	The description matches the changeset and explains the oracle weighting bug fix and test update.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/oracle-weighted-standard-deviation

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[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
x/oracle/types/ballot.go	71.42%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[Copilot]

Pull request overview

This PR fixes oracle reward-band inflation caused by calculating ballot standard deviation using an unweighted variance (and dividing by vote count) while the median is stake-weighted. The updated calculation weights each squared deviation by validator power and divides by total voting power, aligning dispersion with the stake-weighted median.

Changes:

• Update ExchangeRateBallot.StandardDeviation to compute a stake-weighted variance (power-weighted squared deviations / total power).
• Add a safeguard returning zero deviation when total ballot power is zero.
• Update the standard deviation unit test to assert the new stake-weighted result.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
x/oracle/types/ballot.go	Switch standard deviation computation to power-weighted variance and divide by total voting power.
x/oracle/types/ballot_test.go	Update expected standard deviation value to match the new stake-weighted calculation.
CHANGELOG.md	Document the oracle standard deviation fix in the Unreleased “Fixed” section.

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