This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
More so than most shows, the fans of "ER" definitely notice
when the series airs a rerun, due especially to the show`s dedicated
following and serial story nature. I think sometimes fans have a
tendency to exaggerate when it comes to the reruns; generally, "ER"
shows about the same number of new episodes as other shows. The first
season of "ER" was comprised of twenty-five episodes, which is slightly
above normal. The second through fourth seasons of "ER" consisted of
twenty-two episodes when finished, which is very normal; it is expected
that season five and beyond will follow suit.
It may be that "ER" shows an unusually high number of reruns
because the show is rarely pre-empted for other shows. In nineteen
months on the air, "ER" has only been pre-empted five times. Hence,
when other shows are replaced by specials and special broadcasts of
other series` (such as the frequently-pre-empted "NYPD Blue"), "ER"
may be showing yet another rerun. "ER" is a proven ratings winner
even during rerun periods, so NBC doesn't mind programming them.
Another slight difference may be due to what is known as
"sweeps" months: February, May, August and November, when the ratings
battle is especially intensive. "ER"'s producers are especially
mindful of sweeps periods, and perhaps more so than some other shows,
elect to pack these months with new episodes, thus thinning out the
crop throughout the remainder of the season.
*This, of course, applies only to first-run "ER", and only to NBC
(for the most part). "ER" on US TNT is all reruns, all the time--on purpose.
See section 5.11 for more on TNT.