Homeland Security and Uniform Priors

Now that the German Constitutional Court has judged that the so called “Vorratsdatenspeicherung” – the data retention for six months of all phone calls and e-mails to prepare against potential terrorist threats – to be inconsistent with the German constitution, discussions will heat up again regarding the best or at least necessary means to protect against potential terrorist actions. The same court has already ruled out to use dragnet investigation except for the case of an “actual threat”.


Things like dragnet investigations and passenger screening at airports are actually pretty much relevant for the application of statistical methods and data analysis tasks. Whereas for dragnet investigations we rely on the fact that “ordinary citizens” will somehow “live” in the central 99% percent of some multivariate distribution (and someone who prepares a plot will stick out as a multivariate outlier), airport passenger screenings make the odd assumption that all passengers pose the same threat as they are all processed the same way.

The luckily unsuccessful “Christmas Bomber” raised the question again, of whether or not to adopt the Israeli airport security model. This model heavily relies on the personal judgement of the (psychologically) trained screening officer, who looks at the passenger and asks a set of well prepared (from a passenger’s view apparently unstructured) questions. Depending on your answers and reaction (and of course other hard facts), you may walk straight through or miss your flight due to heavy further questioning and examination. (I am very much in favor of this system as the few times I flew to Israel so far, I apparently looked that harmless that I always walked right through – which was actually the right decision of the officers …)

There are certainly many technical issues that need to be addressed if (bigger) airports around the world would try to use this model, but it seems to be odd to assume that every passenger has the same a-priori probability to be a security problem, i.e., the white haired granny from Iowa and the 30 year old arabic male who departed from Syria – they are certainly equally treated by our constitutions, but boarding a plane is still no constitutional right.


  1. float says:

    From how you describe it the Isreali system also puts the same a-priori probability on every passenger, however, they seem to have a reasonable procedure to update these probabilities after an interim analysis. It’s a bit like a response adaptive design. And i wouldn’t dare say that the prior probabilities of the passenger from Syria is that much different from that of the granny from Iowa to justify racial profiling (both are extremely low). If either gets nervous after a couple of well designed questions it might pay of to look closer.
    You are right, we need a system that gathers useful information which allows for an efficient process to update these prior probabilities. However, I don’t believe that much can be learned from data that is automatically collected prior to when the passenger enters the airport.

  2. martin says:

    I agree that there is not too much information we can gather before we actually get to question the passenger. Nonetheless, airlines can, e.g., exchange historical travel data which might help getting a better first evaluation that can then help to select the skill level of the questioning officer.

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