1. A method of compressing a data set comprising a plurality of entries, each of the plurality of entries having a string, the string formed from characters, each character having a value and an ordered position in the string, with one character being the beginning of the string, such that each string has one or more prefix portions, each prefix portion containing a portion of the string having one or more characters at consecutively ordered positions in the string including the character at the beginning of the string, the method comprising: a) determining a plurality of indicators, for each of the plurality of entries, representing the number of entries in the data set with a string having the same prefix portion as the sting of the entry, with each of the plurality of indicators for each of the plurality of entries determined for a prefix portion of the entry having a different number of characters; b) producing a plurality of aggregated indicators, each aggregated indicator representing a combination of the plurality of indicators determined for one of the plurality of entries; c) selecting an entry of the plurality of entries based on the aggregated indicators; and d) compressing the data set using the selected entry.