After using instantcert "only" for the history 2 test I would concentrate 65% of my effort at minimum on the 1900 to 1950 period. There were very few questions on the last 20 years and very few on pre-1900. I have yet to use ant other resource other than instantcert for my 11 passed clep courses to date and I would certainly tell you though that at times I have seen questions I was unprepared to answer, because instantcert didn't cover them. Yet, my average on tests is about a 60 and Iam comfortable with that result.