The prime rate is defined by The Wall Street Journal (WSJ) as "The base rate on corporate loans posted by at least 75% of the nation's 30 largest banks." It is not the 'best' rate offered by banks.
HSH uses the print edition of the WSJ as the official source of the prime rate. Many (if not most) lenders specify this as their source of this index.
The prime rate does not change at regular intervals. It changes only when the nation's "largest banks" decide on the need to raise, or lower, their "base rate." The prime rate may not change for years, but it has also changed several times in a single year.