I do not know who makes these types of decisions. I have read in the past on this board that all radio stations in a given market, collectively decide which geographical areas should be included in the ratings reporting surveys.
For example, Nashua, NH is included in the Boston TV market, while with radio, it is not.
The main criteria for establishing a radio market is the composition of listening in each individual county as well as the integration of factors such as commuting within those counties. It is not the same factor as the OMB and the Census use for Metropolitan Statistical Areas, which are based on economics and not radio listening.
Radio markets are county, not city, based. There are a few split counties (Boston has one of them) but generally whole counties are allocated to radio markets.
A metro survey area can be redefined every year based on listening patterns. A good example is Houston, where counties on the periphery come and go with rather great regularity. This can be done with if rated markets are not contiguous and it's not an issue of taking a county from one market and adding it to another... or absorbing a market within another.
However, when there are adjacent markets, the determining factor is more complicated. For example, in 1980 Miami and Ft Lauderdale were separate markets, but there was a desire by the FM stations to combine to make the metro higher ranked to get more national and regional dollars. The AM stations, which don't cover the full combined area, did not like the idea. Arbitron had subscribers vote and consolidation was approved.
In an opposite situation, it was proposed that the LA market absorb the Riverside/San Bernardino market. But enough stations did not want this to make the initiative fail.
TV markets are very different as they often include vast areas beyond the signal of over the air stations through cable and satellite distribution. They are reach-based survey areas, not signal based. Radio is totally signal based.
So adjustments in non-conflictive counties (that is, they are not being disputed by two adjacent markets) are done purely by Nielsen based on listening. But where a country might move from one market to another, there is a need for current subscribers to participate. The same applies if one market will be absorbed by another. But always the core value is the predominant listening pattern in the county.
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