With all due respect Slava, Prime tends to move in increments of 0.25% more often than not hence the use of the word "usually".
e.g. The last 2 changes in March and April were 0.5%. Before that, the last change >0.25% was in November 2001. Some 22 changes in the rate between then and Jan 2008. All at 0.25% increments.
http://www.fin.gov.bc.ca/PT/bcm/ref/...ricalPrime.pdf
Apologies MikeOxlong.