Problems in that three of the workloads contain no file modification times---how to know if a document is consistent.
Define a cache hit as a match in both URL and size. Using logs with file modification times they found that 1.3% of the references showed the same size, but changes in the modification time.
In VUW logs happened for about 30% of modification time changes. In AT&T and Digital logs happened 20% and 5%.
Note: Although not explicitly stated, a cache hit must be consistent hence they are assuming an "always validate" cache coherency policy.