Featured image of post Understanding Community Notes: Bridging-Based Ranking

Understanding Community Notes: Bridging-Based Ranking


Bridging-Based Ranking is an alternative way to score and rank content by adjusting for user polarization.

The idea was first implemented in pol.is and used successfully by the vTaiwan open citizenship consultation process. The term “Bridging-Based” was introduced in this essay by Aviv Ovadya of the Harvard Kennedy School Belfer Center. UPDATE: A discussion of this article on Hacker News links to additinal blogs and articles.

Diversity of Perspectives

The most successful implementation of Bridging-Based Ranking, X’s Community Notes, explains that the algorithm favors notes that are rated highly by users across a “diversity of perspectives”. But as I show in this article, ratings from users with diverse perspectives are not necessary for a note to rank highly. It is somewhat more accurate to say that the note must be highly rated regardless of diversity of perspective.

The algorithm works by attempting to model why a post receives the ratings it does: how many upvotes are due to users left-wing or right-wing biases, and how many are due to other factors. If a post is only appealing to right-wing voters, and an online forum is dominated by right-wing voters, then that right-wing bias probably explains why it gets so many upvotes. So the algorithm tries to correct for this bias, and estimate how many upvotes a post would receive if there was an equal balance of left-wing and right-wing voters.

Extracting Information

Now why would we want to do this? Why for example would a predominantly left-wing community want to artificially give right-wing opinions more weight, especially if they think their own side is better-informed?

For a fact-checking product like Community Notes, plausible political neutrality may be necessary for public acceptance. But bridging-based ranking has advantages beyond political neutrality: it actually allows us to extract more information from users.

In Community Notes, users rate notes as “Helpful” or “Not Helpful”. If Community Notes was dominated by leftists, what would we learn by the fact that a note received a lot of “Helpful” votes? That it is helpful? Or that it supports a left-wing worldview? Or both? It’s hard to say: perhaps a marginally helpful note that supports a left-wing world view gets more votes than a truly helpful note that supports a right-wing world-view. We can’t tell just from the raw vote counts how “helpful” the note is.

Bridging-based ranking on the other hands let’s us break down the vote counts, attributing some to whatever users think “helpfulness” means and others to polarity. So it is not about giving “both sides” equal weight; by cancelling out the effect of political bias, we can actually extract more interesting information from the users’ votes.

Projection in Opinion Space

The chart below illustrates how this works. This charts shows a subset of notes from the Community Notes public data set, run through my own implementation of the algorithm. The horizontal axis shows the note’s “polarity” – e.g. +1 for right-wing and -1 for left wing – and the vertical axis shows its “helpfulness”. The note’s final score is its vertical component, or its projection on the “helpfulness” axis. The colors of the dots indicate their actual status in Community Notes.

Community Notes Polarity Plot (Notes)

Notice how there is a large spread along not just the the vertical axis, but also the horizontal axis. If want we want to know how helpful a note is, the horizontal axis is just noise. But there is a lot of information along the vertical axis. The vertical component tells us something about the post that users are trying to express through their votes. Separating this component from the horizontal component lets us extract this information, in a sense separating signal from noise.

But what is this information? It is a measure of some feature of a post that users think deserves an upvote independently of their political biases. What exactly this feature is is impossible to say, but presumably it reveals how users interpret the idea of “helpfulness”.

At the end of this article, I include a section with example notes with a variety of factors (Helpful+Left, Unhelpful+Right, Helpful+Neutral, etc).

Why it Works

People are politically biased, but they have other biases, such as the bias towards interesting, accurate, entertaining, or helpful information. They may mostly upvote things that support their political perspective but they will especially upvote things that support their perspective and are actually relevant and factually accurate. And they will tend to downvote notes that support opposing perspectives, but will downvote even more zealously when those notes use false or misleading information.

When bridging-based ranking algorithm dissects users voting behavior and factors out the polarity component, it finds that most users are at least somewhat biased towards helpfulness! You can see this in the plot of a sample of Community Notes users below.

Community Notes Polarity Plot (Users)

There is clump of users in the upper-right quadrant because community notes users are overall right-leaning. But notice also that the helpfulness factor for these users is mostly above zero. They are also mostly biased towards helpfulness. These users are more likely to upvote posts that support a right-wing worldview, and also more likely to upvote posts that are helpful.

Common Ground

This vertical component in these plots represents common ground. It is something users tend to agree on independently of their politics.

In the case of Community Notes, this is presumably some common idea of what constitutes “helpfulness”. But in general what exactly the common ground is depends on the community. Suppose for example there is a forum for Harry Potter fan fiction that unfortunately in recent years it has been overwhelmed by debates about whether J.K. Rowling is transphobic. There is still a lot of good fan-fiction being posted, but the home page is dominated by posts about the controversy.

In this case, the horizontal axis would likely represent the pro- and anti- J.K. Rowling factions, and the vertical axis would represent the common ground of the community: quality Harry Potter fan fiction. Using bridging-based ranking we can in a sense de-polarize the forum, factoring out the effect of polarization and getting back to community’s original essence.

Politics is not the only factor that can divide a forum. Suppose there is a popular forum for posting ridiculously cute pet pics. Sadly, in recent years, two factions have formed: the cat faction and the dog faction. The more extreme cat people mercilessly downvote pictures of dogs (regardless of how cut they are), and the dog people vice versa. Recently, the dog faction has gained the upper hand, and a cat-picture has little chance of making the front page, no matter how frigging adorably it is.

Again, by separating the dog-cat factor from the common ground factor, we can re-focus the community on it’s original purpose: raw frigging cuteness.

Understanding the Algorithm

But how does the algorithm actually work? How does it determine the polarization factor and common ground factor for each user and post?

It actually works using a fairly simple algorithm called Matrix Factorization. Below I will explain how the Matrix Factorization algorithm works, starting with the version implemented by Community Notes and described in the Birdwatch Paper. There is also a good writeup by Vitalik Buterin. In my next post describe my variation of the algorithm that uses multi-dimensional matrix factorization.

A good way of understanding Matrix Factorization is that it is like running a bunch of linear regressions: one for each user and each item.

For example, suppose we have already discovered the polarity factor for each user, and we want to find the polarity factor for each post. A linear regression predicts users’ votes as a function of their polarity factors.

For a highly polarizing right-wing post, the regression line might have a positive slope:

Highly Polarizing Right-Wing Post

        +1   ✕ ✕ ✕ ✕ 
         |    ↗
         |  ↗ 
-1 ______|↗______ +1  User's Polarity Factor
      ↗  |
    ↗    |
  ✕ ✕   -1

In this chart upvotes have a value of +1 and downvotes have a value of -1. All the right-wing users upvoted and all the left-wing users downvoted (as shown by the little ✕s). So the best fit is a line with a slope of approximately +1: the more right-wing the user, the higher the probability of an upvote, and the closer the predicted value is to 1. The more left-wing, the higher the probability of a downvote, and the closer the predicted value is to -1.

Note that there are more right-wing users than left wing users, but it doesn’t make a difference. Even if there were 100 right-wing users and 2 left-wing users, the slope of the best fit would be approximately the same. This is why bridging-based ranking does not favor the majority.

A very polarizing lift-wing post might have a negative slope:

A Highly Polarizing Left-Wing Post

  ✕ ✕   +1    
    ↘    |     
      ↘  |    
-1 _____↘|________ +1  User's Polarity Factor
         |  ↘
         |    ↘
            ✕ ✕ ✕ ✕

For a completely non-polarazing post, on the other hand, the slope would be zero:

A Non Polarizing, “Good” Post

✕ ✕     +1     ✕ ✕ ✕ 
  →  →  →|→  →  →  
-1 ______|________ +1  User's Polarity Factor
       ✕ |       

This is a good post. Not just because the upvote probability is independent of the user’s politics, but because this post receives mostly upvotes – the intercept is above zero. This post has some quality that users of this forum are looking for.

Now, suppose there is a post that looks like this:

A “Good” but Polarizing Post

   ✕    +1   ✕ ✕ ✕ ✕
         |  ↗ 
-1 __ ↗__|________ +1  User's Polarity Factor
    ↗    |
  ↗      |
✕ ✕ 

This post has a positive slope, so it is clearly very polarizing. But the positive intercept means that voting behavior for this post cannot be explained entirely by politics. There is also a component that makes users more likely to upvote it independently of politics.

The Intercept is Common Ground

So the intercept represent “common ground”. It represents something about a post that causes users to upvote independently of politics that cannot be explained entirely by users’ polarity factors.

The Intercept is not the Average

We might suppose that the last post above will receive more upvotes than downvotes because it has a positive intercept. But this is not necessarily the case. It depends on how many left-wing and right-wing users there are. The intercept is not the average: a post can have a positive intercept even though it receives more downvotes than upvotes, or it can have a negative intercept even though it receives more upvotes than downvotes.

What a positive intercept does tell us is that this post would receive more upvotes than downvotes if there was an equal balance of left and right-wing users.

It also tells us how users would hypothetically vote if they were all totally a-political. In such a hypothetical world, the only thing influencing users votes is some common-ground factor that aligns with the intent of this particular community, attracting upvotes independently of politics.

Matrix Factorization

Okay, so we have used regression analysis to find the polarity factor for each post (the slope) of the regression line. But in order to do these regressions, we first need to know the polarity factors for the users.

But how do we find these?

Well, if we knew all the posts’ polarity factors, we could use regression analysis to estimate the probability that a user upvotes a post as a function of the polarity factors of the posts. The slope of the regression line would then be the user’s polarity factor. The regression line for a very right-wing user, for example, might look similar to that for a very right-wing post.

A Right-Wing User

        +1   ✕ ✕ ✕ ✕ 
         |    ↗
         |  ↗ 
-1 ______|↗______ +1  Post's Polarity Factor
      ↗  |
    ↗    |
  ✕ ✕   -1

But we seem to have a chicken-and-egg problem, where we can’t find the polarity factors of users unless we know the polarity factors for posts, and vice versa.

However, the Matrix Factorization algorithm solves this by discovering the polarity factors (and intercepts) for every user and every post all in one go.

It does this by using a single equation to estimate the probability that user $i$ upvotes post $j$:

$$ ŷ_{ij} = w_i×x_j + b_i + c_j $$

Here $w_i$ is the user’s polarity factor, $x_i$ is the post’s polarity factor, $b_i$ is the user’s intercept, and $c_j$ is the post’s intercept.

It then simply finds a combination of values for every $w_i$, $x_j$, $b_i$, and $c_j$ that best fits the data – that produce values for $ŷ_{ij}$ that are closet to the actual values of users vote ($y_{ij}$). This is usually done using a variant of the standard gradient descent algorithm.

The polarity factor the algorithm discovers doesn’t necessarily correspond exactly to politics, or cat-dog preferences, or any measurable quantity. It may be a linear combination of factors. But whatever it is, it represents some latent factor of users and posts that does a good job predicting their votes.


One of the reasons for my interest in bridging-based ranking is that I think it may be a critical part of a social protocol for a self-moderating community. Without it, user polarization will tend to lead to either suffocating uniformity or least-common-denominator mediocrity. Bridging-based ranking can be used in any forum with high entropy (lots of downvotes) as a way to identify posts with posts with high Information Value based on the common-ground factor.

In my next article, I discuss ways that this algorithm can fail, and introduce an improved implementation of the algorithm that users 2-dimensional matrix factorization.

Example Notes

The tables below include examples of notes with various combinations of factors (Helpful + Left, Unhelpful+Left, etc.).

Helpful + Left

The screencap in this image is not real and originated from a satirical website. CNN aired no such report about Musk "threatening free speech" and the chyron has been digitally altered to add the text. https://apnews.com/article/fact-checking-040398325285-0.530.5note url
“Floyd died of a lack of oxygen from being pinned to the pavement with a knee on his neck, not from drug use. A jury unanimously agreed” https://apnews.com/article/fact-check-george-floyd-kanye-west-police-397984860325 Testimony under oath indicated even a "healthy person…would have died". https://www.bbc.com/news/world-us-canada-56670912.amp “it wasn’t drug use, heart disease nor an agitated state” https://www.pbs.org/newshour/amp/nation/toxicologist-testifies-that-drugs-and-heart-disease-did-not-kill-george-floyd-0.430.57note url
The full quote is: "the idea that we’re going to send in offensive equipment and have planes and tanks and trains going in with American pilots and American crews… That’s called World War III" https://www.whitehouse.gov/briefing-room/speeches-remarks/2022/03/11/remarks-by-president-biden-at-the-house-democratic-caucus-issues-conference/ The US agreed to send tanks (equipment) to Ukraine, not troops. https://apnews.com/article/russia-ukraine-military-technology-germany-7b63726a3935da3f03424e98e52a85b2-0.380.63note url
This autopsy was released in 2020, the Medical Examiner Dr. Andrew Baker testified that “subdual, restraint and neck compression” was the main cause for Floyd’s death. https://www.reuters.com/world/us/arrest-videos-undercut-derek-chauvins-murder-trial-defense-pathologist-tells-2021-04-09/ Readers should also note the autopsy this page is sourced from concluded Floyd’s death was a homicide. https://www.npr.org/2020/06/01/867219130/george-floyd-independent-autopsy-homicide-by-asphyxia-0.610.38note url
The fentanyl overdose deaths Greene refers to occurred in July 2020 during the presidency of Donald Trump. Joe Biden was not in public office when the Kiesling brothers died. https://www.detroitnews.com/story/news/local/oakland-county/2023/02/28/southern-border-crisis-michigan-mother-rebecca-kiessling-testifies-sons-fentanyl-overdose-deaths/69953671007/-0.470.51note url
Marsha Blackburn was one of 34 Republican Senators that specifically requested David Weiss be extended special counsel protections and authorities to conduct the Hunter Biden investigation. https://www.cornyn.senate.gov/wp-content/uploads/2022/09/Hunter-Biden-Special-Counsel-Letter-FINAL-2.pdf-0.490.48note url
Community Notes requires agreement from contributors of differing perspectives, as such it is highly resistant to gaming. The entire Community Notes algorithm and data is open source, and can be reviewed by anyone by following the links on the page below: https://communitynotes.twitter.com/guide/en/under-the-hood/note-ranking-code-0.720.31note url

Helpful + Neutral

While Microsoft has substantial investment in OpenAI, Microsoft does not own OpenAI, Inc. nor OpenAI Global, LLC.. https://blogs.microsoft.com/blog/2023/01/23/microsoftandopenaiextendpartnership/ https://en.wikipedia.org/wiki/OpenAI-0.000310.65note url
The post contains part of a digitally altered, cropped and horizontally flipped photo of Waiola Church in Lahaina, Havaii engulfed in flames, dated Aug. 8, 2023 https://newsroom.ap.org/editorial-photos-videos/detail?itemid=da5b172b94a944979645ee7a614a1c78&mediatype=photo.0.000880.74note url
This tweet misrepresents the statements made by Dr Dre, with the real quote being: “Hip hop is evolving, if you don’t like it, don’t listen to it…I’m never going to hate on it” Link to Tweet with Video of Interview: https://twitter.com/dennis_k_g/status/1680919911537991680?s=460.000760.64note url
During ApeFest, attendess suffered from unprotected exposure to UV lights, leading to skin and eye injuries such as sunburn, eye pain, and temporary blindness https://www.theverge.com/2023/11/6/23948464/bored-ape-nft-event-eye-injury-sunburn-uv-exposure-0.0010.64note url
That "cruel" stork needs to get rid of weaker chicks in order to ensure the survival of only the stronger ones. "This behavior occurs in times of food shortage to reduce brood size and hence increase the chance of survival of the remaining nestlings." https://doi.org/10.3161%2F068.037.02070.000620.48note url
An online pay-per-view event cannot sell-out, this is for marketing to encourage people to buy the event.0.000610.47note url
Cigarettes are bad for you. Source: the FDA. https://www.fda.gov/tobacco-products/public-health-education/health-effects-tobacco-use0.00170.74note url

Helpful + Right

Dan O’Dowd owns a competing company writing self driving software. https://www.ghs.com/products/auto_adas.html Dan’s previous attempt to show FSD will "run down a child" was debunked. https://electrek.co/2022/08/10/tesla-self-driving-smear-campaign-releases-test-fails-fsd-never-engaged/ Another user performed a test and it worked as expected. https://www.teslarati.com/teslas-fsd-beta-stops-for-kids/ Tesla’s FSD has over 55 million miles driven w/o any reported injuries.0.980.53note url
"Gender Queer", the book shown in the photo, features sexually explicit material. This book contains visual depictions of oral sex, masturbation and adult sexual contact with a minor. https://en.m.wikipedia.org/wiki/Gender_Queer https://a.co/d/2NsDwZD0.890.54note url
The FBI’s statement is misleading. On May 18, 2023, former FBI agent Steve Friend testified before the House Judiciary Committee that he and others were directed to surveil and document parents attending school board meetings. Source: https://www.c-span.org/video/?528159-1/fbi-employees-testify-weaponization-committee https://twitter.com/realstevefriend/status/16791831234083553300.850.56note url
A video showing the altercation has no apparent cheering and shows multiple bystanders trying to restrain Neely after witnesses claim Neely was erratic and verbally aggressive. https://nypost.com/2023/05/02/shocking-video-shows-vagrant-being-choked-to-death-on-nyc-subway/amp/ https://www.washingtonpost.com/nation/2023/05/03/ny-subway-chokehold-passenger-dies/0.870.55note url
Twitter CEO Elon Musk confirmed that this statement is false and that the suicide prevention feature has not been removed. https://twitter.com/elonmusk/status/1606631724922056704?s=46&t=77JgLCd2bpOdYI6NVuTikQ0.90.52note url
Soros donated 1 million to the Color of Change PAC, the largest individual donation it received in the 2022 election cycle, days after it endorsed Bragg for district attorney and pledged more than $1 million in spending to support his candidacy. https://www.cnbc.com/amp/2023/03/22/trump-indictment-alvin-bragg-ties-to-george-soros-examined.html0.890.53note url

Unhelpful + Left

There’s no evidence Hamas killed them and the person who documented the video confirmed it was Palestinians fleeting to the South as instructed by Israel and Israel killed them while they were evacuating. www.palestine.com-0.67-0.45note url
MEMRI is an organisation founded by a former member of the Israeli intelligence service and has been accused of misinterpreting statements to suit a pro-Israel, anti-Arab narrative. https://www.theguardian.com/world/2002/aug/12/worlddispatch.brianwhitaker-0.52-0.56note url
Musk has a history of posting content that spews hatred towards Jews. This is nothing new. It contributes to antisemitism and has been called out by numerous leaders. https://www.washingtonpost.com/arts-entertainment/2023/05/16/george-soros-elon-musk-adl/ Antisemitic language and content on this site has increased since he took over. https://fortune.com/2023/03/22/antisemitic-speech-twitter-surges-after-elon-musk-takeover-research/amp/ https://www.timesofisrael.com/israel-accuses-elon-musk-of-stoking-antisemitism-on-twitter-after-soros-remark/amp/ https://m.jpost.com/diaspora/antisemitism/article-721724/amp https://www.theatlantic.com/ideas/archive/2023/05/elon-musk-george-soros-anti-semites/674072/-0.63-0.44note url

Unhelpful + Neutral

Elon Musk is an unfunny retard. https://www.thesun.co.uk/news/18976146/elon-musk-school-tesla-child-dad/amp/-0.00045-0.78note url
Elon does not determine the eternal destination of trolls, Jesus Christ does based on their acceptance of His atoning blood shed on their behalf at the cross. https://www.biblegateway.com/passage/?search=Matthew%2025%3A31-46&version=NKJV-0.00061-0.72note url
Jack may be hallucinating on peyote.-0.00041-0.56note url
NNN for NNNs. 🇿🇦🤡0.0015-0.59note url
what a POS-0.0024-0.66note url
It was not an uncommon phenomena that 90s Eurodance groups would frequently and almost indiscriminately change out cast members. In this case, DJ Crazy Times’ female partner is changed in this music video for the upcoming song.0.004-0.48note url
Elon is a man, and errare humanum est.-0.013-0.76note url

Unhelpful + Right

Criticism of George Soros is not a "conspiracy theory," and Musk’s opinion that Soros’ behavior is bad for civilization is a reasonable disagreement. There are countless other examples of misinformation in this thread; this is one example of many. https://www.wsj.com/articles/why-i-support-reform-prosecutors-law-enforces-jail-prison-crime-rate-justice-police-funding-11659277441 https://www.city-journal.org/article/george-soross-bad-bet0.53-0.79note url
By any measurable objective this is incorrect. Trump rebalanced foreign policy to counter China, achieving energy independence, record gains in employment and real wage growth, stemmed illegal immigration and started a global movement to legalize being LGBTQ, etc. https://www.presidency.ucsb.edu/documents/fact-sheet-president-donald-j-trump-has-delivered-record-breaking-results-for-the-american0.79-0.45note url
Fact check: False. Donald Trump was voted America’s greatest president. https://www.irishpost.com/news/donald-trump-voted-americas-greatest-president-2227710.59-0.59note url
Just like Elon Musk, President Trump is making history and changing the world. He was unfairly banned from this site. Having him back can only mean good things for X. Here we go, folks. Its happening!0.6-0.57note url
If the ‘counter protest’ was described as ‘right wing’ & ’extreme’, then this particular protest as pictured could rightly be described as ‘antisemitic’ or ’extreme left wing’ in the interests of balance as opposed to having no label applied by the Metropolitan Police. https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.isdglobal.org/explainers/far-left-antisemitism/&ved=2ahUKEwjC94_khb2CAxWfWUEAHUHPBjYQFnoECA4QAQ&usg=AOvVaw2P8BsRcFaoUOW1cdjX33wP0.55-0.61note url
The EU has presented a list of allegations against X. Thierry himself has been using X almost every day, to publish his own releases and like posts praising his “intellect” . This would make him an accessory in aiding and abetting these all edged serious injustices. https://x.com/thierrybreton/status/1735685982006006137?s=460.89-0.35note url
A lot of salty libs are trying to add note to this tweet, lol 😂 This tweet doesn’t need any note because it’s correct and is just the truth. Cope with it libtards, go back on Reddit 😂0.75-0.43note url
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