( CNN ) -- The FBI and the CIA are being criticized for not keeping better track of Tamerlan Tsarnaev in the months before the Boston Marathon bombings . How could they have ignored such a dangerous person ? How do we reform the intelligence community to ensure this kind of failure does n't happen again ? It 's an old song by now , one we heard after the 9/11 attacks in 2001 and after the Underwear Bomber 's failed attack in 2009 . The problem is that connecting the dots is a bad metaphor , and focusing on it makes us more likely to implement useless reforms . Connecting the dots in a coloring book is easy and fun . They 're right there on the page , and they 're all numbered . All you have to do is move your pencil from one dot to the next , and when you 're done , you 've drawn a sailboat . Or a tiger . It 's so simple that 5-year-olds can do it . But in real life , the dots can only be numbered after the fact . With the benefit of hindsight , it 's easy to draw lines from a Russian request for information to a foreign visit to some other piece of information that might have been collected . Opinion : Agencies often miss warning signs of attacks . In hindsight , we know who the bad guys are . Before the fact , there are an enormous number of potential bad guys . How many ? We do n't know . But we know that the no-fly list had 21,000 people on it last year . The Terrorist Identities Datamart Environment , also known as the watch list , has 700,000 names on it . We have no idea how many potential `` dots '' the FBI , CIA , NSA and other agencies collect , but it 's easily in the millions . It 's easy to work backwards through the data and see all the obvious warning signs . But before a terrorist attack , when there are millions of dots -- some important but the vast majority unimportant -- uncovering plots is a lot harder . Rather than thinking of intelligence as a simple connect-the-dots picture , think of it as a million unnumbered pictures superimposed on top of each other . Or a random-dot stereogram . Is it a sailboat , a puppy , two guys with pressure-cooker bombs or just an unintelligible mess of dots ? You try to figure it out . It 's not a matter of not enough data , either . Piling more data onto the mix makes it harder , not easier . The best way to think of it is a needle-in-a-haystack problem ; the last thing you want to do is increase the amount of hay you have to search through . The television show `` Person of Interest '' is fiction , not fact . There 's a name for this sort of logical fallacy : hindsight bias . First explained by psychologists Daniel Kahneman and Amos Tversky , it 's surprisingly common . Since what actually happened is so obvious once it happens , we overestimate how obvious it was before it happened . We actually misremember what we once thought , believing that we knew all along that what happened would happen . It 's a surprisingly strong tendency , one that has been observed in countless laboratory experiments and real-world examples of behavior . And it 's what all the post-Boston-Marathon bombing dot-connectors are doing . Before we start blaming agencies for failing to stop the Boston bombers , and before we push `` intelligence reforms '' that will shred civil liberties without making us any safer , we need to stop seeing the past as a bunch of obvious dots that need connecting . Kahneman , a Nobel prize winner , wisely noted : `` Actions that seemed prudent in foresight can look irresponsibly negligent in hindsight . '' Kahneman calls it `` the illusion of understanding , '' explaining that the past is only so understandable because we have cast it as simple inevitable stories and leave out the rest . Nassim Taleb , an expert on risk engineering , calls this tendency the `` narrative fallacy . '' We humans are natural storytellers , and the world of stories is much more tidy , predictable and coherent than the real world . Millions of people behave strangely enough to warrant the FBI 's notice , and almost all of them are harmless . It is simply not possible to find every plot beforehand , especially when the perpetrators act alone and on impulse . We have to accept that there always will be a risk of terrorism , and that when the occasional plot succeeds , it 's not necessarily because our law enforcement systems have failed . The opinions expressed in this commentary are solely those of Bruce Schneier .
FBI , CIA criticized for not keeping better track of Tamerlan Tsarnaev before attack . Bruce Schneier : Connecting the dots seems easy in hindsight , but in real life , it 's not . He says there are an enormous number of potential bad guys . Schneier : If a terrorist plot succeeds , it does n't mean law enforcement systems failed .