1. A system for classifying digitized data words into one or more of several categories each defined by a group of digitized file words and comprising: a plurality of groups of comparison means with each group constructed to compare each received data word with each file word in a different one of said groups of file words to produce comparison pulses in response to each comparison of said received data word with a file word; means for generating a variable threshold signal; means for comparing each comparison pulse with said threshold signal; means responsive to said threshold signal and each of said comparison pulses individually to change the amplitude of said threshold signal to a value at which not more than one comparison pulse from each group of comparison means will exceed the amplitude of the threshold signal in a given polarity direction to produce a comparison match; and means for polling each group of comparison means to identify each file word producing a comparison match.