This is a ranking function with ties. Tied items have a rank of the mean of the Ordinal Rank of the tied items.
([Standard Rank] + [Modified Competition Rank]) / 2
Inputs and Setup:
Standard Rank: An instance of Standard Rank computed using an "Advanced" field from best to worst rank.
Modified Competition Rank: An instance of Modified Competition Rank computed using an "Advanced" field from worst to best rank.
Partitioning and Addressing:
Note that the two component table calculations run in different directions. See http://en.wikipedia.org/wiki/Ranking for more details. Values of 5, 3, 3, 2 would produce ranks 1, 2.5, 2.5, 4.
Adopted from James Baker: http://community.tableau.com/thread/109043