1. Apparatus for generating a binary superimposed code word representative of a record containing units of information from which one or more attribute values may be derived, comprising: means for assigning a unique intermediate code to each of said attribute values; means for converting each of said intermediate codes to a corresponding binary code word having a total of b bits of which exactly K bits are ONES; and means for combining said binary code words to form said superimposed code word, wherein said converting means includes: (a) means for mapping said intermediate code word into an integer value lying within a predefined range; (b) means for generating a particular sequence of pseudo-random numbers in response to said integer value; (c) means for scaling each of said pseudo-random numbers in said sequence to a value between 0 and b-1; and (d) means responsive to the value of said scaled numbers for determining the positions of said K bits within said binary code word.