"Section Chief: You dispatched field agents without understanding the base rate theorem!? You're a loose cannon, McGillicutty...
McGillicutty: ...I get results!
Section Chief: Mathematically, you shouldn't. Ergo, you don't."
>Nothing you do with a filter changes the base rate of terrorist plots, which is extremely low.
In that case, I imagine the base rate of 'post-successful terrorist act plotting' is higher than 'pre-successful terrorist act plotting,' which would eliminate your argument.
Also, what you would be studying is the TYPE of terrorist activity not total plot numbers e.g. the stages of activity. If a successful terrorist attack was pulled off not to long ago in the past then that would call for an increase in the base rate of the TYPE of plotting. More terrorists would be more invigorated to pursue/look into similar methods as their next plot.
Maybe the cross-section of the plotting-type-pivot is what you could match against?
The simplest way to express my facepalm is: you are tremendously oversimplifying things.
McGillicutty: ...I get results!
Section Chief: Mathematically, you shouldn't. Ergo, you don't.
Filtering to target at high-risk sub-groups - which, among themselves, have different base rates than the general population- is entirely effective.
It is precisely the reasoning under which medical screening - ineffective, generally, due to the base rate - is aimed at, say, "Men Over 50", where the base rate grows sufficiently large to make screening worthwhile.
As for your filtering observation, yeah, I have no idea if he just mis-stated his position, is using different definitions, or today just can't into math.
The second is worth investigating, and in the interest of teaching others I'll provide a helpful link:
http://en.wikipedia.org/wiki/Base_rate_fallacy
It's unfortunate, because that article and the base rate article have some contradictory conclusions--but, the linked article shows its math.
What's really troubling me here though is that you seem unwilling even as a thought experiment to entertain the notion that this sort of behavior could be purposeful by the government. In many, many prior discussions you've held this view, you've advocated for stripping rights, you've advocated for screwing over people scraping public unauthenticated APIs, you've advocated a great many things against the general notion of the public good, of privacy, and of exploration.
Perhaps come down here and--in the name of intellectual honesty--try and understand what exactly has so many of these people bothered. As it is, due to your choice of message and presentation, you often come off as at best aloof and at worst willfully ignorant.
I think one place to start moving forward is to acknowledge that you really have no idea what I do or don't believe.
A spokesman for the FBI told to the Guardian on Thursday that its investigators were not involved in the visit, but that "she was visited by Nassau County police department … They were working in conjunction with Suffolk County police department."
It is entirely plausible that searches for "pressure cookers" and backpacks are some of several factors used to flag suspects. There is almost certainly a mandate to prevent pressure cooker bombings. I would guess they allotted some resource budget to that task and then simply created search criteria that narrowed the set of cases into one that could be investigated using those resources. I am pretty certain they didn't make any calculations of posterior probabilities. There aren't nearly enough samples to make any kind of meaningful statistical assessment of efficacy, even if you believed that that was the primary decision making tool of the security agencies.
Assuming the story is true the NSA or FBI probably flagged these individuals using a number of factors (search terms, "anti-government" writing, etc) but not enough to merit use of limited federal resources. Still they hit on several risk factors so local police are informed, they may have additional information about these people and have boots on the ground to keep on eye on them (if something did happen then NSA and FBI could wash their hands: "We told the local police, they did nothing"). The comparatively unsophisticated local police, partly motivated by the same fear of being responsible if something did happen, then ham-fisted the investigation.