ACBL pair games on BBO are stratified.
This means that when each pair game starts, players are assigned into one of three "strata" (A,B,C).
The game ultimately produce three sets of winners -- one for A, another for B, another for C.
How do we determine which strata is appropriate for each player? We begin by assigning each player a computed "stratification" score (higher is better). This score is assigned based on several factors including ACBL points, BBO points, BBO "Star", and a couple of other things.
We then take the top third of the field and put it into the "A" stratum. We take the next third and put it into "B". The last third goes to "C". We then evenly divide the A, B and C players across the various sections and directions (NS or EW).
Thus, strata are relative, and depending on the nature of the field, a player might find himself in B, C, or A.
At the end of the game, a pair who is in B has 2 chances to win (A, B). A pair in C has 3 chances ( A, B, C). Masterpoint awards reflect this. Winning in A yields more points than winning in B. Winning in B yields more points than winning in C.
At the end of the day, the stratifications attempt to level the playing field by allowing "C" and "B" players to be compared to other "C" and "B" players (while allowing them to win the "A" if their game is good enough). Stratification never lowers the points you might win, and sometimes raises them.
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