Smith & Wilson B 
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Smith and Wilson's evenness index B (Smith & Wilson, 1996) is defined as:
where ni is the number of individuals in species i; nj is the number of individuals in species j and S is the total species number.
This index is based on the variance in abundance. The variance is calculated using log abundances which means the index examines proportional differences between species. The variance obtained is multiplied by the factor 2/pi arctan() to give an index with the range 0 to 1 with 0 representing minimum evenness.
