Don't confuse Nielsen's DMA with the Direct Marketing Association.
Neilsen Media Research breaks the 210 television markets in the country into Designated Market Areas (DMAs) based on population. Neilsen's DMA list shows Pittsburgh at DMA #22 (when I worked at WPXI it was at at 18). Youngstown comes in at 102. A lot of people working in television mesaure their career goals/success and pay scale/benefits based on the DMAs. DMAs are often seen as the career ladder you climb in the local TV scene.
Arbitron manages the same kind of market ranking system for radio stations, but instead of DMA's you refer to ADIs (Area of Dominant Influence).
I want to stress to my students how important it is to understand these industry terms and not to mix them up in conversation. Such a snafu reflects poorly on your comprehension of how the business works. Using DMA and ADI correctly promotes yourself to potential employers as being industry savvy.
So, when you start developing short and long term career goal statements, make sure you build in something about what DMA or ADI you aspire to break into as an entry level employee out of college and what DMA or ADI you'd like to reach in the next 5-10 years.
While the lists are widely discussed via online resources, they are proprietory information owned by those two companies. However, publications such as the broadcasting yearbook are available-- McGill Library maintains copies of Broadcasting & Cable's Annual Yearbook as a reference resource.