Anonymous Data Collection: Lessons from the A-Rod Affair

February 19, 2009 at 2:24 am Leave a comment

Recently, the Alex Rodriguez steroid controversy has been in the news. The aspect that interests me is the manner in which it came to attention: A-Rod provided a urine sample as part of a supposedly anonymous survey of Major League Baseball players in 2003, the goal of which was to determine if more than 5% of players were using banned substances. When Federal agents came calling, the sample turned out to be not so anonymous after all.

The failure of anonymity here was total–the testing lab simply failed to destroy the samples or even take the labels off them, and the Players’ Union, which conducted the survey, failed to call the lab and ask them to do so during the more than one-week window that they had before the subpoena was issued.

However, there are a number of ways in which things could have gone wrong even if one or more of the parties had followed proper procedure. None of the scenarios below result in as straightforward an association between player and steroid use as we have seen. On the other hand, they can be just as damaging in the court of public opinion.

  • If the samples were not destroyed, but simply de-identified, DNA can be recovered even after years, and the DNA can be used to match the player to the sample. You might argue the feds can’t easily get hold of players’ DNA to run such a matching, but once the association between drug test result and DNA has been made, it is a sword of Damocles hanging over the player’s head (note that A-Rod’s drug test happened six years ago.) The trend in recent years has been toward increased DNA profiling and bigger and bigger databases, and unlabeled samples therefore pose a clear danger.
  • If the samples are destroyed, and the test results are stored in de-identified form, anonymity could still be compromised. A drug test measures the concentrations of a bunch of different chemicals in the urine. It is likely that this results in a “profile” that is characteristic of a person–just like a variety of other biometric characteristics. If the same player, having stopped the use of banned substances, provides another urine sample, it is possible that this profile can be matched to the old one based on the fact that most of the urine chemicals have not changed in concentration. It is an interesting research question to see how stable the “profiles” are, and what their discriminatory power is.
  • Even more sophisticated attacks are possible. Let’s say that participant names are known, but other than that the only thing that’s released is a single statistic: the percentage of players that tested positive. Now, if the survey is performed on a regular basis, and a certain player (who happens to use steroids) participates only some of the time, the overall statistic is going to be slightly higher whenever that player participates. In spite of confounding factors, such as the fact that other players might also drop in and out, statistical techniques can be used to tease out this correlation. 

    This might sound like a tall order at first, but it is a proven attack strategy. The technique was used recently in a PLoS Genetics paper to identify if an individual had contributed DNA to an aggregate sample of hundreds of individuals. 

    I performed a quick experiment, assuming that there are 1,000 players in the sample, of which 100 participate half the time (the rest participate all the time). 5% of the players dope, and each player either dopes throughout the study period or not at all. Testing is done every 3 months; the list of participants in each wave of the survey is known, as well as the percentage of players who tested positive in each wave. I found that after 3 years, there is enough information to identify 80% of the cheating players who participate irregularly. (Players who participate regularly are clearly safe.) 

    [Technical note: that’s an equal error rate of 20%; i.e, 20% of the cheating players are not accused, and 20% of the accused are innocent. There is a trade-off between the two numbers, as always; if a higher accuracy is required, say only 10% of accused players are innocent, then 65% of the cheating players can be identified.]

  • When applicable, a combination of the above techniques such as matching de-identified profiles across different time-periods of a survey (or different surveys) can greatly increase the attacker’s potential.

The point of the above scenarios is to convince you that you can never, ever be certain that the connection between a person and their data has been definitively severed. Regular readers of this blog will know that this is a recurring theme of my research. The quantity of data being collected today and the computational power available have destroyed the traditional and ingrained assumptions about anonymity. Well-established procedures have been shown to be completely inadequate, and it is far from clear that things can be fixed. Anyone who cares about their privacy must be vigilant against giving up their data under false promises of anonymity.

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I'm an assistant professor of computer science at Princeton. I research (and teach) information privacy and security, and moonlight in technology policy.

This is a blog about my research on breaking data anonymization, and more broadly about information privacy, law and policy.

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