(6)
where m is the number of attributes. The function da(x,y) returns a distance between the two attribute values
x and y using one of two functions (defined below), depending on whether the attribute is nominal or numeric.
Many data sets contain unknown input values which must be handled appropriately in a practical system. The
function da(x,y) therefore returns a distance of 1 if either x or y is unknown. Other more complicated methods
have been tried, as in [14], but with little effect on accuracy. The function H is similar to that used in [4],
except that it uses VDM instead of an overlap metric for nominal values and normalizes differently