How frequently a CISSP revision changes ?

karthikaravind

When I browsed through, I came to know that recent CISSP version was live in April 2015

1. How frequently CISSP version changes ?
2. When was last revision prior to April 2015 ?
3. What is the current revision number ?
4. I see that new version of various study materials are being released by the authors on an yearly basis - is it mandatory that we have to read through the latest edition even if the recent CISSP revision change was a few years ago ?