1974-75 Top 30 -- Gunsmoke Ends a 20-Year Run, An Unmatched Record for 35 years

Written Itching Off Shag Ratings Carpets by Bridger Cunningham

Ratings Data prior to 1984 is scarce, to TV Ratings Guide will post what is available and may update overall network ranks and sitcom scorecards when available.

CBS marked its 20th anniversary in 1st Place, and sadly had to end a ratings hit of a same age as Gunsmoke fired its last Nielsen bullets.  The last testament to high-rated westerns ended in 28th place, a respectable end for a then-record-setting show.  Consequently, The Simpsons surpassed its "scripted" record in 2009, albeit for comedy.  And Law and Order limped off the 2010 schedule, having matched Gunsmoke.  Elsewhere, Mannix also ended a long-run on CBS with 8 seasons, and a rough season left NBC with only 7 top 30 entries.  ABC again rested in what appeared to be a permanent home in 3rd place, but the dynamics changed the following season.  

Socially-relevant sitcoms painted much of the chart, led by another landmark year for All in the Family.  The Norman Lear hit now had a third spinoff as The Jeffersons made a mid-season debut and landed in 4th Place.  Mary Tyler Moore also reproduced as spinoff Rhoda outrated its parent show.

Source -- http://fbibler.chez.com/tvstats/by_5_yr_period/top_programs_1970-1975.html