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Anonymity of Netflix Prize Dataset Broken 164

Posted by Zonk
from the there-are-degrees-of-anonymity dept.
KentuckyFC writes "The anonymity of the Netflix Prize dataset has been broken by a pair of computer scientists from the University of Texas, according to a report from the physics arXivblog. It turns out that an individual's set of ratings and the dates on which they were made are pretty unique, particularly if the ratings involve films outside the most popular 100 movies. So it's straightforward to find a match by comparing the anonymized data against publicly available ratings on the Internet Movie Database (IMDb) (abstract on the physics arxiv). The researchers used this method to find how individuals on the IMDb privately rated films on Netflix, in the process possibly working out their political affiliation, sexual preferences and a number of other personal details"
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Anonymity of Netflix Prize Dataset Broken

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  • by tygerstripes (832644) on Tuesday November 27, 2007 @10:29AM (#21491743)
    Who goes out of their way to rate "Anal Whores 3" online?
    • Re: (Score:2, Funny)

      Bill Clinton?
    • by mh1997 (1065630) on Tuesday November 27, 2007 @10:46AM (#21491931)

      Who goes out of their way to rate "Anal Whores 3" online?
      The good thing about porn flicks, as a general rule, is that they're too bland to have really bad plots. The search for good dialogue strays too far off the beaten path established by the social mores of the target market, be that old men, college students, or perverts out on dates. There are pornos with solid plots, just rarely pornos with complicated plots.

      What they generally aren't is full of capers designed by crackheads in search of sexual relief, or a dominatrix dying to destroy the gold market with a Da Vinci alchemy machine only a cat burglar from Hoboken could steal.

      Yes, the plot of Anal Whores 3 is as convoluted as it is kitschy. Mercedes and Veronica Diamond forcibly enlist the help of happy-go-lucky and half-a-second-out-of-prison pizza delivery man Hawk (Peter North) to steal the pieces to a machine that turns lead vibrators into gold. Hawk isn't halfway to a cup of coffee with his wise cracking cohort, Tommy (Johnny Cockring) when he finds himself back in the burglary game. Casing out a heist he meets nun/professional patron of the arts/double agent/love interest Jessie Jane (vows of bestiality can put the kibosh on even the best of cinematic love interests). When you throw in a CIA agent (Dick Coburn) and a couple of double dildos, you've managed to make the world's most convoluted porno....

      • by styryx (952942) on Tuesday November 27, 2007 @11:01AM (#21492083)
        That's the plot of Hudson Hawk. Good flick.
        • by mh1997 (1065630) on Tuesday November 27, 2007 @11:56AM (#21492849)
          If I had mod-points, I'd mod you up insightful. I didn't think someone would spot where I copied the review from so fast.
        • Re: (Score:3, Informative)

          by caluml (551744)
          Yeah, I really liked it too. Quite surreal, funny, and the VHS copy I bought was purchased in Kazakhstan, so it has Russian subtitles, just to add to the weirdness. And when you tell people it has Bruce Willis in it, they're surprised.
          Andi McDowell imitates a dolphin in it too.
        • The Fifth Element. I think it's because it's a mixed-genre movie. Not action, not comedy, somewhere in between.
          Both star Bruce Willis, interestingly enough.

          "Hey mister, are you gonna die?"
          "Do you know what it's like to be called Chlamydia for a year?"
          "You are a slender reed compared to that guard"

          Both HH and 5E are in my top 10 movies. And the commentary on Hudson Hawk is great - they talk about how they hired the narrator from Rocky & Bullwinkle, so that you'd know the tone they were taking. Fun stu
        • That's a side of Sandra Bernhard I didn't want to see.

          "Looks like Bunny's got today's balls balls."

      • Re: (Score:3, Funny)

        by Jtheletter (686279)
        The search for good dialogue strays too far off the beaten path established by the social mores of the target market

        I see what you've done there..... ;)
      • by SkyDude (919251)
        While I'm sure your posting is sort of serious, I can't help chuckle at the thought of being serious about a form of "entertainment" where every five minutes or so, someone's tongue or penis is in one of someone else's bodily orifices.

        It's kind of ............surreal.

        • Re: (Score:3, Insightful)

          by Minwee (522556)
          Is that any more surreal than a form of "entertainment" in which people get shot at or blown up every five minutes or so?
  • Probabilities (Score:5, Insightful)

    by dj245 (732906) on Tuesday November 27, 2007 @10:31AM (#21491765) Homepage
    The researchers used this method to find how individuals on the IMDb privately rated films on Netflix, in the process possibly working out their political affiliation, sexual preferences and a number of other personal details"

    This is a loaded statement. The most you can determine is that if a person likes movie A, B, C and D but hated E and F, there is a higher probability they are a guy. If they liked Z but didn't like X, there is a higher probability they might be a republican than not. You're still anonymous.

    Unless, of course, you're one of the three people that liked "Glitter". Then I think they might have something on you.
    • Re:Probabilities (Score:4, Insightful)

      by Se7enLC (714730) on Tuesday November 27, 2007 @10:39AM (#21491861) Homepage Journal
      I think they're on to something here. They cracked the anonymity by using the public movie ratings (and the dates those ratings were made) as a key. If the user has rated enough movies (especially some of the less-often-rated movies) you can uniquely identify which user they are. Once you know which user they are, you have now connected a username to the list of private ratings.

      Now, they go one step too far to say that you can determine anything but movie preferences out of a movie rating list. Just because somebody liked or disliked brokeback mountain doesn't mean they are gay or straight, just like their opinion of michael moore movies doesn't give political affiliation.

      It will tell you what movies they rented, though, and some people might not be happy having their movie-renting history publicly available.
      • Re: (Score:3, Insightful)

        by Dare nMc (468959)

        one step too far to say that you can determine anything but movie preferences out of a movie rating list.

        also your taking a aggregate of the household. So a household (will call them Chen'ys) had a gay kid, and the devil living in the same house with a Saint... good luck figuring out when the gay kid updates the queue, and when the Wife, or the Devil is at the keyboard.
        • Re: (Score:3, Interesting)

          by coolGuyZak (844482)
          Some tech-savvy households may enable profiles on Netflix, enabling each person to track their likes & dislikes independently. (I did this for my GF, who has wildly disparate tastes from me). I'm not sure what effect that would have on the data. It'd certainly be neat if the scientists could differentiate between individual and multiple users using a particular profile.
    • From the paper:

      First, we can immediately find his political orientation based on his strong opinions about "Power and Terror: Noam Chomsky in Our Times" and "Fahrenheit 9/11." Strong guesses about his religious views can be made based on his ratings on "Jesus of Nazareth" and "The Gospel of John". He did not like "Super Size Me" at all; perhaps this implies something about his physical size? Both items that we found with predominantly gay themes, "Bent" and "Queer as folk" were rated one star out of five.

    • Re:Probabilities (Score:5, Insightful)

      by Chapter80 (926879) on Tuesday November 27, 2007 @11:15AM (#21492277)
      I think you're missing the point.

      If you rate a handful of movies on ImDB, under the persona "MyNickname12345" and that can be traced to your personal MySpace page, you have made that choice. No problem.

      If you then submit 100 movie ratings to Netflix, assuming that it is PRIVATE information that will not be linked back to you, and then Netflix releases the data to the public, now the 100 movies can be correlated to you, and your name can be revealed. Researchers have shown how PRIVATE DATA released to the public can be linked to already public information. PROBLEM!

      • by timeOday (582209)
        I think this research may be unethical. If I argue persuasively that somebody could stalk somebody by following them around with binoculars and laser microphones, that's one thing; but if I prove my point by DOING that, it's different.
        • by FLEB (312391)
          I'd say it depends on what you do with the knowledge once you have it. They seem to be going about it in a responsible manner-- releasing only enough relatively tame data to prove the viability of the process.
          • by plover (150551) *

            Releasing only ... tame data

            They released a lot more than some data: they published the algorithm. Anyone is free to write their own implementation of it. And anyone who is participating in the Netflix prize already has a copy of the database.

            However, I do agree with you that they went about it in a responsible manner. They revealed it. Without their insight, we might have continued living in ignorance that some "unknown adversary" (external to Netflix) is already correlating our movie rental habit

    • Now do you see the POWAH inherent in that knowledge?
  • Privacy is becoming a fleeting thing in this interconnected world. Perhaps we should reanalyze our perspective on it all?
    • Re: (Score:3, Informative)

      by SatanicPuppy (611928) *
      Perhaps if we're obscure and pretentious enough, no one will want to spy on us! Brillant!

      The world changes. Learn to live with it.
      • by phobos13013 (813040) on Tuesday November 27, 2007 @12:15PM (#21493125)
        Actually TFA seems to suggest that the more obscure and pretentious we are, the easier it is the track us. If we become homogeneous drones voting on the top 100 films, we are safe! Even so, I don't plan to become a homogeneous drone...
      • by mollymoo (202721)

        The world changes. Learn to live with it.

        What a defeatist attitude. Why not try to change it into the world in which you want to live? You can be damn sure someone else is trying to change it into the world they want to live in, which may well be at odds with the world you want to live in, so if you just "learn to live with it" you're setting yourself up to be shat on from a great height.

        If you don't like the idea of personal information being mined in this way, talk to your friends and write to your

    • But then we'd have to re-analyze capitalism itself, an I don't think society is ready for *that* rich people would simply pay for organizations to falsify their data, it would be one sided.
  • Do what now? (Score:5, Insightful)

    by faloi (738831) on Tuesday November 27, 2007 @10:32AM (#21491785)
    It doesn't sound like the anonymity of the prize set was broken through any fault of NetFlix. It sounds like some sampling of users made the mistake of rating movies on a site where the info is publicly available, and a site where it's not. All they did was correlate the two.

    So the lesson is, basically, don't post stuff that you don't want to be public to a website that makes it public, right? This is sounds roughly like blaming the DMV for figuring out a car owners likely political leanings by the bumper stickers on their car.
    • by Dare nMc (468959)

      was broken through any fault of NetFlix.

      just because someone choose to go public with liking "The Rise of Theodore Roosevelt" doesn't mean they should know that the company will take some seemingly private data linking you to really likeing "brokeback mmoutain", and the series "The L word" and publicize it later. and that the combination of your post, and the combination now violates netflix's privacy policy (in spirit)
      IE they say they will only disclose "on an anonymous basis" anything but your reviews.

    • Re:Do what now? (Score:5, Insightful)

      by IBBoard (1128019) on Tuesday November 27, 2007 @11:08AM (#21492169) Homepage
      Exactly - all they did was found that there was a correlation that might mean that the people are the same on IMDB and NetFlix. There's also the possibility that they're different people and that they just voted similar on different places.

      Besides, this all relies on people voting for a) really obscure films so they can be easily identified and b) voting similarly or identically on lots of films so that they can get a better idea as to whether it is the same person based on them liking the same films the same amounts.

      Just because two people from two different data sets both like (and are the only people in the data sets to like) lemon and custard jam as well as peanut butter with chips doesn't mean they're the same person, it just means they could be the same person and have similar tastes in obscure foods.
      • Re:Do what now? (Score:4, Insightful)

        by Peter Mork (951443) <Peter.Mork@gmail.com> on Tuesday November 27, 2007 @11:14AM (#21492253) Homepage

        Exactly - all they did was found that there was a correlation that might mean that the people are the same on IMDB and NetFlix.

        Caveat: I haven't had a chance to pore over the statistical calculations. However, the paper notes that their similarity measure was 38 standard deviations from the norm. Assuming the math is valid, this seems on par with a DNA test, which also provides a correlation. I wouldn't be so quick to dismiss the results until you can find a serious methodological problem.

        • Re: (Score:3, Insightful)

          by IBBoard (1128019)
          While yes, they did get a very perfect match on that record, the line about it is:

          ...our algorithm identified the records of two users the Netflix Prize dataset with eccentricities of around 28 and 15, respectively.

          Granted they went for a small number of IMDB users due to their TOS, but that's still a tiny fraction. They mention finding a perfect match in IMDB and 1/8th of the NetFlix database towards the start of the report (although the sentence is a bit clunky and unclear). If that's their general accura

      • Re:Do what now? (Score:4, Informative)

        by arvindn (542080) on Tuesday November 27, 2007 @01:08PM (#21493827) Homepage Journal
        "Besides, this all relies on people voting for a) really obscure films so they can be easily identified "

        not true -- obscure films help a little bit but not too much. we put up a recent draft of our paper in which the dependence on obscure movies is much reduced.

        "b) voting similarly or identically on lots of films so that they can get a better idea as to whether it is the same person based on them liking the same films the same amounts."

        again not true at all. one of the main claims of our paper is that our method is tolerant to an INCREDIBLE amount of noise. we have the math to back this up.

        --Arvind Narayanan

      • by Dread_ed (260158)
        Decoding and privacy aside, this analysis of movie ratings might be a good way to find twins seperated at birth or to match people up for dates.

        NetFlix.dating anyone?
    • Re: (Score:3, Insightful)

      by JPMH (100614)
      Their lesson is that it can take surprising little public information to identify you.

      For example, ratings on a scale of 1-5 for 2 movies, and a knowledge of when they were seen to within 14 days, was suffiecient to identify the complete data histories of 40% of the Netflix clients. As the authors say, that's the kind of information cooleagues give out every day around the water cooler.

      Repeating the experiment with a knowledge of 8 movies, 6 hits in the database would be sufficient to identify the per

      • by drew (2081)
        ...99% of clients who had also rated the same movies on IMDB
        (unless I greatly misunderstand their method)

        I've you've only ever given your movie ratings to NetFlix, then they still have no way to correlate that with any other source. (And even then, all they've shown is that User #1234 in the Netflix List is the same person as RandomPseudonym582 on IMDB, which personally I don't find to be terribly interesting.)
    • Re: (Score:2, Informative)

      by roadkill_cr (1155149)
      True, but in the real world, it's not as simple as that. There are cases of publicly available databases that you gave no permission to grant access to (for example, AOL's release of their search queries). There are other cases when a database has restricted access, but a person with access to it takes it and uses it in comparison with other databases available. Hackers are always a trouble; since some have gotten into such "secure" areas as the CIA and IRS, what's to keep them from potentially getting i
    • Re: (Score:3, Insightful)

      by yali (209015)

      So the lesson is, basically, don't post stuff that you don't want to be public to a website that makes it public, right?

      Nope, it's more complicated than that.

      Suppose that you want to keep your political attitudes private -- for whatever reason, you decided it's nobody else's business. On IMDb, linked to your real identity, you only rate movies with non-political content, which you don't mind anybody knowing your opinion about. On Netflix, you believe that your ratings will be kept private, and you want to

  • by CastrTroy (595695) on Tuesday November 27, 2007 @10:36AM (#21491815) Homepage
    Seems like it was only broken because the identity of the people was posted somewhere else, along with the ratings. My only question is how they connected the rankings on Netflix, to the rankings on IMDB. Does Netflix take the liberty of submitting all the users rankings to IMDB for them, and also include their name with this data? If you just have anonymous dataset A, with anonymous dataset B, you could match up users from both and figure out which person in A is the same person in B, but you still wouldn't know who the person is. However, if you now have dataset B be not anonymous, then it's not too difficult to compare movie ratings and find out who the people are.
    • They are just saying it is likely a person rated a movie on Netflix and IMDb at roughly the same time. That is the correlation which is need to connect the anonymous with the publicly posted information.

      While I do rate a few films on IMDb I usually do them in batches, where on Netflix I rate the movie as soon as I'm finished viewing it. So the time link wouldn't be there between my two accounts.
    • What NetFlix did that was stupid was include the names of the movies in their dataset. There was no need for this for the prize (unless anybody was using the names for prediction I suppose), anonynous identifiers would have been okay.
      • If a person liked season 1 of Stargate: SG1 it would be a good idea to recommend season 2 to them. Goes for sequels too. So yeah, titles are needed a little bit.
        • And methinks you need more than just titles as well. Ex: Joe rents "Shaun of the Dead", and hates it. It would be a good idea to not suggest "Hot Fuzz", as they share many of the actors, directors, etc.
        • Not really. They just needed to provide sequel and tie-in information in their dataset. The exact relationship doesn't matter. And you can't get that info from the titles necessarily, anyway. I mean, how is a text-based algorithm going to know that "Serenity" is the movie tie-in to the series, "Firefly?"
          • by darthflo (1095225)
            If the algorithm would be going to do more than just correlate past ratings with ids of other movies and sequels/tie-ins of other movies, more data about the movie is required. Crew data would be extremely useful here. If somebody liked Live Free or Die Hard and Perfect Stranger, The Fifth Element may also be of interest because of the common lead actor.
            Including such data in the example sets would in turn allow to determine the correlation between movies and their internal id number pretty quickly. Even i
      • by jfengel (409917)
        They were hoping that by giving the name of the movie they could pull in other sources of data (RottenTomatoes, IMDB) to make better guesses. They were seeking more than just better curve-fitting of the data points. If nothing else, the IMDb's huge pile of data points of ratings gives you a lot more fodder for your collaborative filter, but only if you can tie in the names of the movies.

        But yeah, that introduces a data leak.

        • They were hoping that by giving the name of the movie they could pull in other sources of data (RottenTomatoes, IMDB) to make better guesses.

          Are you sure? Are people using data from outside the training set? Because if what you say is true then they're essentially asking people to use some kind of probabilistic record linkage to include external databases, which would automatically include the personal identifiers. This would be highly dubious behaviour.

          • by jfengel (409917)
            They don't need to know this particular user. The collaborative filtering system works by comparing your pattern to other patterns. Find somebody (anybody) with a similar pattern and then use their ratings on movies you didn't rate. You can use other Netflix users (which is what Netflix has been doing all along) but you can increase your pool with the IMDB. It makes it more likely to find somebody who has similar ratings to yours overall and has seen some movie that you haven't rated.

            Which almost makes
  • did it work? (Score:3, Interesting)

    by Speare (84249) on Tuesday November 27, 2007 @10:39AM (#21491859) Homepage Journal

    The researchers used this method to find how individuals on the IMDb privately rated films on Netflix, in the process possibly working out their political affiliation, sexual preferences and a number of other personal details

    {tongueincheek}Yeah, but the question is, will knowing those personal facts generate better movie recommendations?{/tongueincheek}

    When there's a significant prize at stake, researchers can try all sorts of slimy tricks to win. (I'm not saying that's the motive behind this report, but there are many "researchers" going for the prize.) And when there's significant profits at stake, a corporation will damn-fire-certainly use whatever means they can use to maximize those profits, regardless of whether it might be "ethical."

    • by johnbr (559529)

      And when there's significant profits at stake, a corporation will damn-fire-certainly use whatever means they can use to maximize those profits, regardless of whether it might be "ethical."
      Here, let me fix that for you:
      When there's significant profits at stake, individual humans will damn-fire-certainly use whatever means they can use to maximize those profits, regardless of whether it might be "ethical".
  • by Anonymous Coward
    For those who haven't rated movies on IMDB, such as myself - and I imagine a large proportion of subscribers.
  • by Anonymous Coward on Tuesday November 27, 2007 @10:54AM (#21492005)
    There are two things going on here. One, many people are asking how you could identify any personal information about people based on their movie preferences. The answer is data-mining. Very sophisticated techniques exist to do things exactly like this, i.e. take a data set and find out about the people.

    The second problem is that by deanonymizing the NetFlix data, you can start to cheat on the NetFlix prize. The requirement to win $1 million is that your recommendation engine is 10% better than the one they are currently using. However, if you can learn the exact preferences of some users in the dataset (i.e. by finding the rest of their ratings on IMDB) then you can hardcode that into your recommendation engine and get the recommendations for these users exactly right. This can boost your score even though your actual system is no better than the existing one. This is known as over-fitting to the data.

    Finally, this paper is over a year old. Can we please have some new news?
    • by _14k4 (5085)
      I don't know, about your second problem. They separate the dev/test data from the quiz data - and even that is halved into two sections. With the intent to stop a "hill climb" in the results. What says that the dataset used in developing the code is a subset of the data used in the test to find the winners?
      • by darthflo (1095225)
        They don't have to hardcode data from the test set into their algorithm. Possible solution goes like this: 1) Have IMDb rating dump available (if this is trivially possible, retrieve it at runtime) 2) Load quiz data, create profiles 3) Create proviles out of the IMDb data (if not already done) 4) Correlate profiles from 2 and 3 5) Use some (e.g. Netflix') statistical method to determine the probability of user x linking film y 6) Override values from 5 with x's actual IMDb data where possible Ta-daah, you
        • by _14k4 (5085)
          So.. wanna join my netflix team? :P

          I found that this application may be a good way to tinker with some math, statistics, and such while I self learn - with no intention of submitting anything real to netflix itself.
          • by darthflo (1095225)
            Sure, where do I join? :)

            Sounds like a nice opportunity to learn something. I don't have any idea on how to implement any of the stuff I mentioned before, though.
            • by _14k4 (5085)
              I simply created my own team over at netflixprize.com to get access to the datasets. I don't plan on submitting anything worthwile to the project, or rather the contest - however, I do plan on learning a lot about more methods of datamining. What I do here at work isn't mining, but simply reporting. (Albeit on tables with millions of records, still.)
  • by Thanshin (1188877) on Tuesday November 27, 2007 @10:55AM (#21492011)
    Every time you feel the need to vote 10 in Glitter, also vote 10 to The Godfather.
    Every time you cheer for Brokeback Mountain, also put a 10 in Huge Knockers MXII.
    Every time you want to express your love for Dersu Uzala, vote a 10 in Spice World, with added commentaries.

    That way, everybody will know you're a security conscious computer scientist. Or a squizophrenic moron.
    • by Belial6 (794905)
      Or, you are married. I know, queue the jokes about Slashdot readers not having wives.

      Really, one of the biggest problems with Netflix ever getting good recommendations is that they are not trying to make recommendations for individuals. They are making recommendations for a group of people who's tastes my not cross over at all. You joke in your post about what movies get a 10 (well, 5 anyways), but would it seem unreasonable that a family of 4 could come up with those very ratings?
  • by call -151 (230520) * on Tuesday November 27, 2007 @10:55AM (#21492013) Homepage
    The summary is somewhat misleading- the only accounts that can be identified are those that belong to people who also rate on IMBD and who have thus chosen to make at least some of their ratings public. If person X rates 1000 movies on Netflix and has made 20 or so ratings on IMDB publically available, then it is possible to infer with some small uncertainty which of the anonymized individuals in the NetFlix database they are. Thus you have possibly figured out their ratings of the other 980 movies they rated for Netflix but did not post on IMBD. Interesting, but not earth-shattering or a serious breach of privacy, I would say.
    • Interesting, but not earth-shattering or a serious breach of privacy, I would say.

      And who exactly are you to say so?
      Because it isn't a Credit Card # or SSN it isn't serious?

      A) Some people would rather go to jail or commit suicide than admit to something embarrassing they'd rather keep private. Privacy isn't (just) about hiding (illegal) things from the Government.

      B) Demographic information is something you can never take back and can never change.
      At least I can get a new credit card & SSN.

      • Re: B, you can usually change any sort of non-biological (and, using extreme measures, some biological ones too) demographic information about yourself. There's nothing that says you can't suddenly turn from liberal to conservative or vice versa, or get married (or turn gay/lesbian), etc.

        OT: is there a way to escape greaterthan/lessthan signs?
        • by Lijemo (740145)

          OT: is there a way to escape greaterthan/lessthan signs?

          apersand-lt-semicolon results in <

          apersand-gt-semicolon results in >

          (no spaces or dashes.)

        • OT: is there a way to escape greaterthan/lessthan signs?

          You have to use the HTML escape codes, which are &lt; and &gt; .
    • For most people, it wouldn't be a serious breach of privacy. However, you can imagine a scenario where it would be.

      Imagine a pastor who uses a recognizable username for many sites, including both IMDB and his church's web forums. He uses Netflix as a way to feed his secret love of movies with sexual content which his church would publicly denounce. Now these researchers could link his username to ratings for all these movies, and post the information online.

      All it would take then is for a curious church mem
  • by puppetluva (46903) on Tuesday November 27, 2007 @11:02AM (#21492089)
    This is total hyperbole.

    All they researchers are saying is that they can deduce some of your preferences based on your other preferences. Of COURSE you can do that, that was the whole point of the contest Netflix put up.

    What they are _not_ saying is that they now know who you are, where you live, or anything uniquely identifying about you. So basically, you are still anonymous.

    I'm starting to tire of news headlines that claim the world is on fire when someone actually just does something slightly derivative from the norm and thinks they are brilliant. The noise from these non-events mask actual brilliant achievements and make it seem that everyone is doing banal work.

    • Re: (Score:3, Insightful)

      by Peter Mork (951443)

      All they researchers are saying is that they can deduce some of your preferences based on your other preferences.

      The researchers are making a stronger claim. They are stating that based on actual public ratings (available from IMDB) they can generate actual private ratings published by Netflix under the guise of anonymity. As the paper notes, someone competing for the Netflix prize could use this data to improve the accuracy of their prediction algorithm. However, the point of this paper is to reveal t

      • by Jay L (74152)
        Great example, Peter Mork.

        In addition, I'd point out that this can probably be generalized to (a) any anonymous data set that's combined with (b) some other non-anonymized data set that will map onto it. Here, we have (a) a sample of anonymized Netflix data and (b) a sample of non-anonymized IMDB ratings. So a lot of the reactions are either "Well, duh, if you post publicly to IMDB, you've posted publicly, so you're stupid!" or "it's only movies, who cares?"

        I care. Not just in the hypothetical of "what i
    • Re: (Score:3, Informative)

      by JPMH (100614)
      Othe the other hand, if somebody *already* knows who you are, the lesson is that it can take surprising little public information to identify your entire history of ratings at Netflix.

      For example, the authors found for 40% of individuals, accurate ratings on a scale of 1-5 for only *two* random movies,together with a knowledge to within 14 days of when they were seen, would be sufficient to identify an individual in the dataset. As they comment, that's the kind of information cooleagues give out every day a
    • by TubeSteak (669689)

      All they researchers are saying is that they can deduce some of your preferences based on your other preferences. Of COURSE you can do that, that was the whole point of the contest Netflix put up.

      What they are _not_ saying is that they now know who you are, where you live, or anything uniquely identifying about you. So basically, you are still anonymous.

      Did you even read the summary?

      They took anonymous ratings & discovered they can link some of them to IMDB usernames. We can argue over whether or not those IMDB usernames are "uniquely identifying" or "anonymous" but they definitely say something about who you are.

      I'm sure a percentage of those IMDB usernames are easily linked to real people through a trivial google search. Does that break this alleged veil of anonymity? Datamining isn't that hard these days.

      • by puppetluva (46903)
        I did read the summary and felt that the IMDB linkage was a real stretch.

        Linkage of that kind is only useful if the user-populations for IMDB commenters and Netflix commenters are the same (at least 50%) and that most people make the same comments and ratings on both systems in the same way _most_ of the time. Chances are that if the populations are _not_ the same and that the commenters don't mostly duplicate their ratings for every movie in each place. . . In that case, you then you probably get more fal
    • by arvindn (542080)
      Netflix claimed that the data is anonymous.

      They said you couldn't identify a person's record in the dataset even if you know some (or all!) of their ratings.

      We showed that that's not true. Even if there's a LOT of noise. That's all there is to it.

      --Arvind Narayanan

  • by SmallFurryCreature (593017) on Tuesday November 27, 2007 @11:11AM (#21492221) Journal

    As far as I know in IMDB you are rating the overall quality of the movie, not I agree with it OR I want to see more like this.

    One example, Shindlers list, great movie, do NOT want to see it again. Same with Grave of the fireflies. Some movies just ain't for multiple viewings. They are my "favorite movies I never want to see again".

    On the other hand I got movies I can watch any day of the week, but that I would NEVER rate as highly. Cannonbal run is one such movie. It watch it far too often, but I wouldn't call it a good movie. You can always fine me ready for a Jacky Chan movie or a spagethi western.

    Is the netflix rating system a "I liked this movie and want to see more like it" system or a "This movie was brilliant and I would highly recommend it too everyone else" type of rating system?

    Granted some people get it confused, probably the same people that use the slashdot moderation system to silence views they don't like, but that only makes basing conclusions on user ratings even more problematic.

    I can rate a movie highly even if I do not agree with it, simply because it is good. And I can rate a movie I really like to watch as crap simply because I know I like watching crap.

    I don't like the godfather movies, I can see they are high quality, I just don't like them. So my rating them would be fairly high as for quality, but low for 'I want to see more like this'.

    I thought that the netflix system was "I want to see more like this" based. Surely nobody is so stupid as to think a quality rating and a "i like this" rating system are the same? Or am I completly in the wrong in seeing a difference between the two? Am I insane in thinking that you can see a movie as being a great artwork and still not liking it or viceversa?

    • by apt142 (574425)
      I would think that "I like this" and "This is a good movie" are two different measurements on a film.

      I must be a pessimist, but I don't believe the average Joe would agree with that statement. I think most people would see the two statements as synonymous. That is, if they even think about the distinction. Mostly I think they'd just grab their "gut" feeling and go with it.

      I suppose we could test the argument by comparing movies that are ranked high on quality with total movie rentals or some other
    • Re: (Score:3, Interesting)

      by xtracto (837672)
      One example, Shindlers list, great movie, do NOT want to see it again. Same with Grave of the fireflies. Some movies just ain't for multiple viewings. They are my "favorite movies I never want to see again".

      Just out of curiosity, why don't you want to see those films again? both of them are really good films and although I would not see them every weekend (as for example Sin City), I enjoy watching them from time to time. The plot is interesting, the photography/drawing is nice and the screen writing is wel
      • The comment "favotire movie I never want to see again" is one I got from a review of Grave of the Fireflies that I just happened to totally agree with. Don't read the reviews, just watch it yourselve and if you are not into Anime just set that aside for the duration of the movie, then ask yourselve again, if you can understand that comment.

        It is powerfull movie, like Shindlers List, but not a happy tale. I am not talking a tear jerker movie here, I am talking a "we will all burn in hell for this" movie. Te

    • Re: (Score:3, Insightful)

      by Danny Rathjens (8471)
      As far as I know in IMDB you are rating the overall quality of the movie, not I agree with it OR I want to see more like this.

      No. You give people way too much credit if you think their ratings on public sites are that nuanced or objective. I think most people just rate things on how well they like it themselves. A significant portion seem to even just give 10s to anything they like, too.

      I also find it amusing how the votes tend to congregate somewhere in the 3rd quartile a bit above average(e.g. 7 on
      • Re: (Score:2, Insightful)

        by ps236 (965675)
        > I also find it amusing how the votes tend to congregate somewhere in the 3rd quartile a bit above average(e.g. 7 on a 1-10 scale) rather than 5.5 where it would be if people ranked things more fairly

        I'm not sure about that. People will tend to watch films they think/hope they will like. So, the ones where they think 'that'll be absolute poop' they won't bother watching, so, hopefully, won't bother rating.

        So, people should rate fewer films as 'poop' than as 'great', because they select only the 'hopeful
      • by ZorbaTHut (126196)
        In addition to the previous comment, when I rate movies I'm usually rating movies that I remember. If a movie is entirely unmemorable, I'm not gonna remember that I watched it and thus I'm not going to rate it.

        That means the above-average movies and the total flops get rated, but not the below-average movies.
    • by arvindn (542080)
      Yes, such differences in meaning exist.

      However, when you're talking about dozens of movies, all you need is a correlation. Our algorithm is powerful enough to tolerate a large amount of noise. If you read the paper, we were able to match up users between imdb and netflix with a very high level of confidence, in the sense that the best match was 15-30 standard deviations away from the second best match. In statistics terms, that's a insanely close match.

      --Arvind Narayanan

    • by coaxial (28297)

      Is the netflix rating system a "I liked this movie and want to see more like it" system or a "This movie was brilliant and I would highly recommend it too everyone else" type of rating system?

      It's both. The system allows users to say how much they preferred a movie. This can then be used to predict what movies a user will prefer in the future. If an unseen movie is preferred by users that have expressed preferences similar to yours, then it will be recommended to you.

      But your question about what the semantics of a rating are is good one. The answer is, we don't really know, and it doesn't really matter from a practical standpoint. People like things of "high quality" whatever that means.

  • ...would be a lot more appreciative of this proof of concept if someone trawled Slashdot threads to see how often you feed trolls by responding to comments with a "-1" rating... :P
  • Wait - you mean if I enjoyed a movie with a gay theme, people are going to assume I'm gay?

    Anyone think the IMDB rating of Brokeback Mountain is going to plummet dramatically. (It is 7.8 today)

    And of course, if it does, we will be able to correlate the timing of the sudden drop with the publishing of this slashdot article, allowing us to link the slashdot readership with imdb users. Now we have your Netflix ratings, IMDB ratings, AND slashdot postings all correlated...
  • It should really read "non-anonymous ratings of movies in IMDB gives away your movie preferences".
    Because really, thats all the mat their "research" has.

    And in fact, if you rate movies in IMDB, and your handle can be tracked, who needs the netflix data?

    This whole thing is a non-issue, and the paper is so content-slim i doubt it will be accepted anywhere (well, maybe "new scientist" will print it...)
  • To the issue of your anonymity being shattered, puh-lease. If you post information in a public forum such as IMDB and it can be correlated to information from MySpace, it wasn't a giant leap into your privacy. It was just gathering already public information. What's the big deal?

    You choose to post that stuff where it could be publicly viewed. The fact that it lines up with data from Netflix only proves that NF did in fact provide a quality dataset. Big deal.

  • by AmiMoJo (196126) <(ten.3dlrow) (ta) (ojom)> on Wednesday November 28, 2007 @10:20AM (#21504043) Homepage
    None of the mainstream media picked up on it, but I remember thinking this sort of thing might be possible with the data lost by HMRC too. I bet Tesco would love to get their hands on it for planning where to put new stores and what to stock etc. Combined with their Clubcard database, of course.

"In matters of principle, stand like a rock; in matters of taste, swim with the current." -- Thomas Jefferson

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