In my board, for your first 3 years of reffing, you write the NFHS test. Then in your 4th year you are allowed to write the IAABO exam. I guess they figure that if you can pass the Fed exam for 3 years you can pass the IAABO one. I can say that comparing this years fed test and this years IAABO test, I wish I wrote the IAABO test, because it looked much easier.