You have to adjust for free float market cap. Companies need to be US incorporated. You have to be aware of share classes. For example both GOOG share classes a

You have to adjust for free float market cap. Companies need to be US incorporated. You have to be aware of share classes. For example both GOOG share classes are in the S&P. When it comes to tiebreak decisions, there may be 5 or so potential adds when there is 1 delete. There may be a focus on evening out sector exposures, preferencing underweight sector adds. (Just my guess)