4. The method of claim 2 , wherein the step of generating, for each item or event, a compilation of data entries that include each item or event comprises the step of: generating a vertical representation comprising a column including a plurality of identifiers, each identifier identifying an item or event in the dataset and a list including for each identifier, a bit vector indicating, for each data entry in the dataset, whether the data entry includes the item or event identified by the identifier.
2. The method of claim 1 , wherein the step of counting each occurrence of each item or event in each data entry in the dataset comprises the step of: generating a count array comprising a first column including a plurality of identifiers, each identifier identifying an item or event in the dataset, and a second column comprising a plurality of counts, each count indicating a number of occurrences of an item or event identified by a corresponding identifier.