In the United States, the sweeps period determines local advertising rates. (National advertising rates are based on Nielsen Ratings.)
Networks consequently ramp up their programming during these four four-week periods in February, May, July, and November. Outside of these peak periods, it is more common to see reruns of popular programs, while most of the more shocking plot elements (which, in turn, cause more viewers to tune in, pushing advertising rates up) happen largely during sweeps. Newscasts often feature sensationalized reports, sometimes turning into bizarre exposés.
In some rare cases (such as N.Y.P.D. Blue and The O.C.), a few shows do not even premiere new episodes until November sweeps, while it is traditional to begin the new American television season in September. Fox is the only network to do this on a regular and constant basis, delaying the premiere of all major shows until November. This is largely due to the network's coverage of the Major League Baseball playoffs and World Series in October.
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