1. A computer-implemented method for estimating a frequency of errors for each of a plurality of data processing operators, the data processing operators performing a data processing operation on a plurality of documents, the method comprising a computer system executing software to effect the steps of: assigning documents to different groups of operators, each group containing at least two operators, each operator belonging to at least two groups; making the documents that are assigned to a group, available to the operators in the group in order for each operator to type data in the documents assigned to the group, the different operators thus generating different versions for each document; collecting the versions of the documents typed by the different operators; statistically analyzing discrepancies between the different versions without human intervention and without comparing to a known transcription of the document; and based on the statistical analysis of documents assigned to groups and based on the assignment of operators to groups, estimating an average frequency of errors, the frequency of errors averaged over the plurality of documents.