The formula for determining Best Picture nominees

The Writers Guild (WGA) has given awards every year since 1948. They initially used a different calendar year for awards than the Academy of Motion Picture Arts and Sciences. (award list)

Further Info About the Formulas and Methods

Statistically speaking, this is not a rule but a formula showing a percentage of likelihood - and yes, rules are made to be broken and formulas are fallible (ie: a 1998 Best Picture prediction of Saving Private Ryan based on DGA+ACE would have been incorrect). And of course, these formulas only work in the years when there is an overlap with the Guild awards mentioned above - which is rarely for DGA+ACE+WGA, but is approaching half the time for DGA+ACE as well as DGA+WGA.

What I was looking for in a formula was a correlation between indicators with large enough sample sizes to be meaningful. While DGA+ACE+WGA has a sample size of 49 (years), there are only eight occasions of overlapping data - however, those seven occasions predict the Best Picture 100% of the time.

I think the stronger indicator might be DGA+ACE though. With a sample size of 49 years (1961-2009), there are 21 occasions of overlapping data (42.9%), with 20 of those occasions correctly predicting the Best Picture (95.2% accuracy). I'll leave it to the mathematicians to determine the statistical significance of these results, although I think DGA+ACE might actually be the strongest indicator I've found.

I didn't include stats from the other guilds in my formulas for these reasons:
1. Except for the MPSE (57 years), the sample sizes are too small: PGA (21) / SAG (16) / ASC (24) / ADG (14) / CDG (12) / MHG (5) / CAS (17) / VES (8).
2. With the possible exception of the PGA 14/21 (PGA overlapping data: 14 Best Pictures out of 21 PGA awards), there was so little correlation of data between Best Picture winners and other Guild winners, there was no further use in studying the possibilities:
PGA - 14/21 (66.7%) [The Hurt Locker / Slumdog Millionaire / No Country for Old Men / The Lord of the Rings: The Return of the King / Chicago / Gladiator / American Beauty / Titanic / The English Patient / Forrest Gump / Schindler's List / The Silence of the Lambs / Dances with Wolves / Driving Miss Daisy]
SAG - 18/80 (22.5%) [Slumdog Millionaire-Ensemble / No Country for Old Men-Javier Bardem / No Country for Old Men-Ensemble / Crash-Ensemble / Million Dollar Baby-Hilary Swank / Million Dollar Baby-Morgan Freeman / The Lord of the Rings: The Return of the King-Ensemble / Chicago-Renee Zellweger / Chicago-Catherine Zeta-Jones / Chicago-Ensemble / A Beautiful Mind-Russell Crowe / American Beauty-Kevin Spacey / American Beauty-Annette Bening / American Beauty-Ensemble / Shakespeare in Love-Gwyneth Paltrow / Shakespeare in Love-Ensemble / Fargo-Frances McDormand / Forrest Gump-Tom Hanks]
ASC - 6/24 (25.0%) [Slumdog Millionaire / American Beauty / Titanic / The English Patient / Braveheart / Dances with Wolves]
ADG - 7/14 (50.0%) [The Hurt Locker / Slumdog Millionaire / No Country for Old Men / The Lord of the Rings: The Return of the King / Gladiator / Titanic / The English Patient]
CDG - 4/12 (33.3%) [Slumdog Millionaire / The Lord of the Rings: The Return of the King / Chicago / American Beauty]
MHG - 1/5 (20.0%) [The Lord of the Rings: The Return of the King / Chicago]
MPSE - 5/57 (08.8%) [Gladiator / Titanic / Braveheart / Patton / Ben-Hur]
CAS - 7/17 (41.2%) [The Hurt Locker / Slumdog Millionaire / No Country for Old Men / Gladiator / Titanic / The English Patient / Forrest Gump]
VES - 1/8 (12.5%) [The Lord of the Rings: The Return of the King]

As for the importance of editing and why it's such a big predictor, start with reading some basics in wikipedia:
"Film editing is an art form which can either make or break a film. A film editor works with the layers of images, the story, the music, the rhythm, the pace, shapes the actors' performances, "re-directing" and often re-writing the film during the editing process, honing the infinite possibilities of the juxtaposition of small snippets of film into a creative, coherent, cohesive whole... Film editors often are responsible for pulling together all of the elements of story, dialogue, music, sound effects, visual effects, rhythm and pace of a film. In the making of a film, the editors usually play a dynamic and creative role." In essence, screenwriting is storytelling during pre-production; editing is storytelling during post-production.

Guild Abbreviations:
ACE (1961-2010) - American Cinema Editors (award list)
DGA (1948-2010) - Directors Guild of America (award list)
WGA (1948-2010) - Writers Guild of America (award list)
ADG (1997-2010) - Art Directors Guild (award list)
ASC (1987-2010) - American Society of Cinematographers (award list)
CAS (1994-2010) - Cinema Audio Society (award list)
CDG (1999-2010) - Costume Designers Guild (award list)
MHG (2000-2004) - Makeup and Hairstylists Guild (award list)
MPSE (1954-2010) - Motion Picture Sound Editors (award list)
PGA (1990-2010) - Producers Guild of America (award list)
SAG (1995-2010) - Screen Actors Guild (award list)
VES (2003-2010) - Visual Effects Society (award list)