Evidence-based airport security?

Backscatter scanners cost $150,000-$180,000 a piece. Congress has appropriated funding for 450 scanners to be placed in the US airports. By my simple back-of-the-envelope calculation we are talking $67,500,000-$81,000,000 of our taxpayer dollars. How do we wrap our brains around the value proposition of this investment?

Well, in order to do this we need to know something about what outcome we are trying to affect and the impact of this machine on this outcome of interest. Seems like the most sensible outcome of interest is deaths averted due to airplane-related terrorist activity. According to Nate Silver at fivethirtyeight.com, in the first decade of the 21st century, factoring out the 9/11 fatalities, there were about 200 deaths related to violent incidents on board of commercial aircraft in the entire decade. So, assuming that the scanners' effectiveness is 100% (that is that it can prevent any violent act aboard an airplane that would result in any number of deaths), the cost to avert one death is $337,500-$405,000 over 10 years (this is not taking into account either inflation or discounting for future events).

If we had a medical technology with the same cost-effectiveness profile, would we think it reasonable to pay for it? More importantly, would we get congressional appropriations to pay for it even in the absence of any effectiveness data? What we know and what we think we know about this are widely divergent. What we know is that the scans can detect certain culprits of potential acts of terrorism. What we do not know is whether this level of detection will indeed result in aversion of death. After all, had we not detected prior to the Detroit flight that Umar Farouk Abdumutallab was a potential threat to US citizens? We had, but the information was not acted upon. Similarly with the scanners, they may have efficacy in detecting the threat, but how this information is utilized is sure to impact their effectiveness as far as the endpoint of interest: death prevention. So, reaching for a new expensive and potentially more invasive technology in this case is a bit like blaming one's failure to treat a cancer detected on a mammogram on unavailability of the more sensitive MRI technology.

There seem to many points of faulty logic in this undertaking of heightened airport security. Not only do we need to re-examine critically our assumptions, but also we have to assess soberly whether the investments we have made to date are in the right places. Remember the cold war? It was all about getting a leg up in the nuclear arms race. "You build a better missile, we will build a missile shield. Then you will build an even better missile that will penetrate the shield." And so on, and so on. It is much like laws, upon which clever corporate lawyers rely for loop holes in favor of their clients' misdeeds.

As much as anyone, I would like international terrorism, just like ventilator-associated pneumonia and other hospital-acquired complications, to become a "zero event". Unfortunately, I am all too keenly aware that wishing something to be true only makes it so in fairy tales. The dearth of evidence to support many of the expensive anti-terrorism interventions is concerning. Poor logic, erroneous assumptions and unjustified inferences have been driving our decisions for too long. The public should demand the same level of evidentiary support for astronomical "anti-terrorism" appropriations that we do for healthcare.