On 3 (taking out tops and bottoms part): BBO uses Cross IMPs. It does NOT use Butler, so there is no average involved. Tthe process is exactly the same as MPs (your result is compared to every other result one-on-one, and assigned a result), but instead of 1/0.5/0, the assigned result is based on the IMP table. Then, all results are added (if you recall the Cavendish scores, they are calculated this way). BBO, after all this, divides that number by the number of comparisons, so the final result looks like something that could come from a Butler process, but it is not. <opinion> Butler is a crutch from pen and paper times to need less math operations to arrive to a firly good result. Cross IMPs are fairer than Butler. With computers doing the math, the crutch is no longer needed. </opinion>