but also delineate which channels will be received by satellite or cable subscribers ("must-carry" rules). These market areas can also be used to define restrictions on rebroadcasting of broadcast television signals. Generally speaking, only stations within the same market area can be rebroadcast. The only exception to this rule is the "significantly viewed" list. Virtually all of the United States is located within the boundaries of exactly one TMA. A similar term used by Nielsen Media Research is the Designated Market Area (DMA), and they control the trademark on it. DMAs are used by Nielsen Media Research to identify TV stations that best reach an area and attract the most viewers. There are 210 Nielsen DMAs in the United States, 70 of which are metered (in other words, viewership in these markets are estimated automatically instead of through the archaic diary system still in use in the smaller markets). TMAs may cover a much larger area than the stations that serve it, especially since the digital television transition. This is particularly true in markets that have hilly or mountainous terrain that is ill-suited for digital broadcasting. In these cases, the outlying areas of a TMA may only be served by cable and satellite, or perhaps by small translators. (There are some cases, such as that of Olean, New York, where a sizable number of independent stations operate, but none carry any major network affiliation unless they operate as translators. Because of this, Olean is considered part of the Buffalo, New York market despite none of that city's major signals reaching the city from 70 miles away.) Conversely, a geographically small market such as Erie, Pennsylvania may have stations where their signal spills well over into neighboring TMAs (most of Chautauqua County, New York, is closer to Erie than Buffalo, but the county is also located within the Buffalo DMA). Arbitron (now Nielsen Audio) also maintained similar areas for television ratings, each called an "area of dominant influence" (ADI), which were first created in 1966. For the 1993-1994 television season, there were 209 ADIs in the continental United States. Arbitron stopped offering a television ratings service in late 1993. Radio Nielsen Audio (previously Arbitron) maintains smaller areas for radio stations; each is called an Arbitron Radio Metro. Whereas a typical TMA may cover ten counties, an Arbitron market generally covers two to four, and a TMA may contain two to four