Voting for the Oscars
One very important aspect in predicting the Oscar winners and especially the Oscar nominations before they are announced, is understanding the Academy componence and voting procedure. AMPAS consists of approximately 6,000 voting members and all eligible members of the Academy may vote in the Best Picture category, while Academy members in 12 branches will nominate achievements in up to an additional 21 categories. Films submitted in the Animated Feature and Foreign Language Film categories are screened and selected by special voting groups of Academy members.
In other words, actors nominate the actors, directors nominate the directors, etc. making the impact of the Guild nominations even more obvious on the outcome of the Oscar nominations since they represent, in principle, the exact set of people that will decide the Oscar nominations in the respective categories.
However, things are not as simple since the Academy has about 1300 actors, while the SAG has over 20 times as many. The 2004 SAG nominations in contrast were determined by a preselected panel of 2100 members.
According to the latest info I could find, the Academy membership breakdown in 2003 was as follows:
5,803 Total voters
1,298 Actors
465 Producers
433 Executives
403 Writers
416 Sound
365 Public relations
366 Art directors
372 Directors
366 Members-at-large
307 Shorts
241 Music composers, lyricists
239 Visual effects
222 Film editors
182 Cinematographers
128 Documentarians