The biggest flaw in our security...

[Snow]

I posted this in the Lounge so it wouldn't vanish too fast, then a mod moved into the GD. And there she went, faster than an Ozymandias thread. Soooo, let's give this a look here.

This is not an exciting topic in that it includes
math, but it shows a basic flaw in our security screening
that I've seen the press address only once. Here goes. Let's
say you want to screen out bad guys. The bad guys occur at a
rate of 100 per million. You have a great screen that catches
99% of the true positives (ie the bad guys) while correctly
labeling 99% of the true negatives (the ordinary travelers).
Here's how that looks tabulated (bear with me; we
epidemiologists love to tabulate stuff).

Truth

Bad Guy Ordinary Traveler Total
----------+-----------+----------------------+------------
Screen | | |
---------+ | |
Bad Guy | 99 | 9,999 | 10,098
| | |
---------+-------------+----------------- ----+------------
| | |
Ordinary | 1 | 989,901 | 989,902
Traveler | | |
---------+-------------+----------------------+------------
Total | 100 | 999,900 | 1,000,000
| | |
| | |

Okay, see the problem? Your screen, which is really
fantastically accurate, has caught all but one of the bad
guys. But it's also labelled 9,999 ordinary travelers as bad
guys - those are your false positives - but to you, based on
your screen, they look just like the bad guys. You have no
way of telling them apart. You can play with the numbers,
base it on passengers travelling by plane per day, change my
assumptions about the proportion who are terrorists, maybe
even see what happens with a not-so-good screen (the false
positive rate gets worse). But this is why we truly are not
any safer than before and why the idea of screening a large
population for a rare condition is pointless in this case.
The foundation of what we're trying to do with airport
security is flawed.

edits to make the table work....

[cosmicaug]

1. Snow.

I thought you died almost 150 years ago.

Epidemiologist Snow wrote

edits to make the table work....

It still doesn't work too terribly well.

[Snow]

2. Arrghh - table re-edit

because I forgot (again) to click the box about plain text
formatting. 
Here we are:

                         Truth
               Bad Guy       Ordinary Traveler   Total
  ----------+-----------+----------------------+------------
  Screen  |             |                      |
 ---------+             |                      |  
  Bad Guy |  99         |       9,999          | 10,098
          |             |                      |   
 ---------+-------------+----------------- ----+------------
          |             |                      |   
Ordinary  |    1        |     989,901          | 989,902
Traveler  |             |                      | 
 ---------+-------------+----------------------+------------
Total     |   100       |     999,900          | 1,000,000
          |             |                      | 
          |             |                      | 

How's that?
(and you got that date of death correct, cosmicaug....nice
catch)