This article is from the NYPD Blue FAQ, by Dave Chapman, Alan Sepinwall and Jeff Knapp with numerous contributions by others.
Bill Clark, [see 2.10 for more info] a retired NYPD detective,
serves as the show's technical advisor to make sure that Milch
& Co. "get things right." If you ever see Clark's name in the
"Story by" credits of a given episode, then one or more of the
cases in that show are based on a real-life event.
While Clark is a real stickler for detail, he also understands
the demands of TV storytelling, which is why the events on the
show often seem time-compressed. Cases that are solved in the
traditional two-day span of an episode often took weeks or
months to play out in reality, even if all the incidents
portrayed in the show happened in real life. (For example, the
story about James getting shot in "E.R." actually happened,
with Clark in the Bobby role as the man who blew the whistle
on the DA's deal with the shooter, but the whole thing took
place over several months.)