Here is a great link on nominal record linkage: http://books.google.com/books?id=9yL5HMBUnFQC&dq=An%20Optimal%20Theory%20of%20Record%20Linkage&pg=PA7#v=onepage&q=An%20Optimal%20Theory%20of%20Record%20Linkage&f=false It's a proceedings from 1985, but in includes a selection of the classical papers on the topic from as early as the 50s. Tom