what has the prime interest rate been for the past 20 years

Here's the data!

The most concise answer to your question, which has a prime rate graph
as well as a table with statistics from 1992 (6.5%) to 2002 (4.75%) is
found here:

http://www.nfsn.com/library/prime.htm#graph

The highest rate was seen in 2000 (9.5%).  The lowest shown is the
current 4.75% rate.


The web site below lists prime rate statistics for the '70s, '80s,
'90s, and 2000s.  Just scroll to the bottom of this page:

http://www.hsh.com/idxhst.html

Other sites have historical data for longer periods of time.   You can
see lists of detailed numbers as well as graphs.

http://www.hsh.com/indices/prime.html

http://www.briefing.com/FreeServices/mtgdata/prm.htm

http://www.federalreserve.gov/releases/h15/data.htm

http://research.stlouisfed.org/fred/data/irates/prime

http://www.ntea.com/MarketData/market_data_detail.asp?ID=32

I used search terms
"prime rate" historical graph
"prime rate" historical chart
"prime rate" historical

That should be enough data to isolate and analyze just about any time
period within those 20 years!