The selectivity ratio (Sr) of a potential key value in a table is a good guideline to its usefulness as an index. If the number of rows, which are uniquely identified by the key, is Rk, and the total number of rows on the table is Rt, then

Sr = 100*(Rk/Rt)

The lower the selectivity ratio, the more useful the key would be as an index to the table. SQL Server stores this ratio for each index. Note that if the selectivity ratio of a key is more than 15%, the optimizer will ignore the index and chose a full table scan.