With the shift to a 64-team tournament in 1999, the NCAA eliminated its former practice of using some procedure to select which conferences were eligible to qualify a team for the tournament automatically. All 29 conferences now send an automatic qualifier (normally, but not necessarily, the winner of a post-season conference tournament). [The requirement that a conference meet certain minimum requirements to be included in this group was not eliminated, however.]
Two systems were used before 1999, both based on the conference's overall RPI (Ratings Power Index) from the previous season.
From 1994 to 1999, the conferences were split into two groups based on the RPI. One group, with the highest RPIs, automatically qualified a team for the tournament. They get a "." in my table below. The remaining conferences, either 8 or 10 depending on how many conferences there were, were paired up (apparently based solely on region) into a set of best-2-of-3 "playin" series whose winner was guaranteed a spot in the tournament. They get a "W" or "L" in the table below depending on whether they won or lost. Those that would have been in a playin series in 1999 get a "*". The number of playin conferences was chosen to keep the total number of slots for at-large teams around 24.
Prior to 1994, conferences were either in or out based entirely on their RPI from the previous season. Again, those that qualified automatically are shown with a "." in the table below, while those left out are given an "x". The number of automatic bids was 24 in 1992 and 1993, but was as large as 26 in 1990 and 1991.
Note that I suppress "Conference" and "Athletic Conference" from the end of official conference names and give the common abbreviation I use elsewhere in square brackets.
year(199x)
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Conference 9876543210 comments
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America East ..W renaming of North Atlantic
American South .. --> merged with Sun Belt
Atlantic Coast [ACC] ..........
Atlantic 10 .W..WW....
Big East ..........
Big Eight ....... --> Big 12 with 4 from SWC in 97
Big South ....W..x..
Big Ten ..........
Big 12 ... merger of Big 8 and 4 from SWC
Big West ..........
Colonial Athl. Assoc. ..........
Conference USA [CUSA] .... merger of Metro and Great Midwest
Diamond x was part of the ECAC
East Coast ... --> Big South (2), ECAC (3)
Eastern Collegiate .... --> separate conference bids in 1994
Eastern Intercollegiate ... --> Ivy (dropping Army and Navy)
Great Midwest xxxx --> CUSA (4), Atl-10 W
Ivy League *WWWWL. was main part of EIBL before 1993
Metro Atlantic *LLLLL used to share one ECAC bid
Metro (or Metropolitan) ...... --> CUSA (5), Atl-10 W, Colonial
Mid-American [MAC] ..........
Mid Continent *WWWWWxxxx
Mideastern [MEAC] *LLLLLxxxx (never in NCAA tournament)
Midwestern Collegiate *LLLL...xx
Missouri Valley ..........
North Atlantic .L.. shared ECAC bid --> America East
Northeast *WWLLW used to share one ECAC bid
Ohio Valley *W.WWLxx..
Pacific Ten [Pac-10] ....... excess teams from N dropped later
Pac-10 North ... --> joined S to form single conf.
Pac-10 South [Six-Pac] ... --> joined N to form single conf.
Patriot League *LLWWL used to share one ECAC bid
Southeastern [SEC] ..........
Southern ...W.W....
Southland ..........
Southwest [SWC] ....... --> Big 12 (4), 2 WAC, 1 CUSA
Southwestern [SWAC] *LLLLLxxxx (last in NCAAs in 1989)
Sun Belt ..........
Trans America [TAAC] .....W....
West Coast [WCC/WCAC] *.WL.W....
Western [WAC] ...W......
==================== ========== =================================
9876543210
* = identified as playin teams for 1999 if expansion not approved
. = automatic qualifier for 1999 and/or previous year
W = qualified by winning playin tournament
L = lost the playin tournament
x = did not qualify for bid
Finally, many thanks to Rick Rollins for some of the conference details and double-checking my results.