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FEED_FAIRNESS.md

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Feed fairness: merit over reach

This package makes For You rank by predicted value for the viewer, not by how large an author's existing audience already is — while keeping user mutes/blocks consistent and stopping viral originals from flooding a slate through many retweeters.

It is not demographic parity, identity quotas, or equal impressions for every account. Low Phoenix quality stays low. Spam and predicted block/mute/report stay negative and are not lifted.

Changes

1. Author-size IPS (ranking residual)

Equal Phoenix scores should not be ordered by follower count. Horvitz–Thompson residual on ln(1 + followers), mean-normalized inside the scored batch.

Full math, knobs, and worked example: MERITOCRATIC_AUTHOR_SIZE_IPS.md.

Code: home-mixer/scorers/author_size_ips.rs.

2. Size-aware out-of-network relief

Discovery for accounts a viewer does not follow is almost entirely OON. The flat OonWeightFactor (default 0.75) stacked on audience-size leakage double-penalizes small creators.

oon' = base + (1 - base) * relief * t

t is 1 at or below SizeAwareOonFollowerFloor (default 1k), 0 at or above SizeAwareOonFollowerCeiling (default 100k), linear between. Default relief = 0.5 → a 500-follower OON post uses 0.875 instead of 0.75. Missing follower counts keep the base tax. Large OON accounts keep the full tax.

Param Default Meaning
rust_home_mixer_enable_size_aware_oon_relief true Master switch
rust_home_mixer_size_aware_oon_follower_floor 1000 Full relief at/below
rust_home_mixer_size_aware_oon_follower_ceiling 100000 No relief at/above
rust_home_mixer_size_aware_oon_relief 0.5 Share of the gap to 1.0

Code: RankingScorer::oon_weight_for in home-mixer/scorers/ranking_scorer.rs.

3. Origin-author diversity

Author diversity used candidate.author_id. A viral original could occupy many slate slots via distinct retweeters, each counted as a first appearance.

With EnableOriginAuthorDiversity (default true), diversity counts get_original_author_id() (retweeted_user_id when present). The second and later appearances of the same original author decay.

Quotes still diversity-key on the quoter: a quote is new content by that author.

Param Default Meaning
rust_home_mixer_enable_origin_author_diversity true Decay on content origin for retweets

4. Mute/block symmetry for quotes and reposts

AuthorSocialgraphFilter already dropped quotes and reposts of blocked authors. Mute only checked the candidate author, so a quote or repost of a muted user could still serve.

Mute now mirrors block for quoted_user_id and retweeted_user_id. Viewer preference is respected whether the muted account posts directly or is amplified by someone else.

Code: home-mixer/filters/author_socialgraph_filter.rs.

What this does not do

  • Does not replace retrieval. A post that never enters the candidate set cannot be residualized. SidTail / post_creation retrieval remain the natural next step.
  • Does not use UserCred / PageRank as a For You weight.
  • Does not soften safety or spam drops. Visibility filtering stays separate from ranking.

References