DHS Doesn't Know or Doesn't Want Us to Know How

DHS doesn't know or doesn't want us to know how effective its border-security efforts are because

A) most of them feel that they really aren't doing that good of a job, but they want to hide that fact to increase their budget.
B) there are too many variables to consider in whether or not something could be considered a preventable terrorist attack.
C) they use a metric of "number of apprehensions" to say they are doing an effective job, which is a measure of activity, not program results.
D) not enough time has passed for them to effectively evaluate most program results.