To answer that question, I found the season averages on each event from everyone headed to Trials. To be fair, season averages may not be the best judge of who will perform the best, but I figured it would be a decent baseline. Falls ARE included in the average, so this is not an average of only hit routines. They're rounded to the hundredth's place.
The cool thing about this 4 person team is that if need be, these 4 girls could fill the 3 up, 3 count quota on each event. Here's how I'd do it.
VT : Wieber (15.85), Raisman (15.58), Douglas (15.24)
UB : Douglas (15.55), Ross (15.28), Wieber (14.85)
BB : Ross (15.1), Raisman (15.03), Wieber (14.85)
FX : Raisman (15.34), Wieber (15.08), Douglas (14.97)
So to figure out which gymnast would fill the fifth spot, I figured out who would add the largest advantage by adding their routines instead of the lowest scoring routine(s) in the lineup above.
To my surprise, the girl who would add the largest advantage was McKayla Maroney who would increase the team average .84 if she vaulted instead of Douglas. Marta's openly said that she doesn't want to take a girl who can only help on one event, but McKayla's average is by far the most helpful to team USA.
Second is Alicia Sacramone whose average is higher than both Douglas on Vault and surprisingly, Jordyn Wieber on beam. (Jordyn's Pac Rim EF fall IS factored into this average though.) Her advantage is .53.
Elizabeth Price's was the third most helpful with her vault average adding .45 to the team total.
Bridget Sloan, Anna Li and Rebecca Bross each could help the team solely on bars, but because they've each had their share of mistakes their advantages each only average out to less than .2.
Kennedy, Sabrina, Nastia and Brenna don't have higher averages on any event than the already chosen four.
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