This is a good paper on this topic: http://dl.acm.org/citation.cfm?id=1555372 http://dl.acm.org/citation.cfm?id=1555372. Another slightly older paper with simi

This is a good paper on this topic: http://dl.acm.org/citation.cfm?id=1555372 http://dl.acm.org/citation.cfm?id=1555372. Another slightly older paper with similar data: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.115.7026&rep=rep1&type=pdf http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.115...