13. The computer-readable medium of claim 11 wherein the first number is a weighted combination of a plurality of the views of the document wherein one or more of the views is weighted using a respective weight based at least in part on a respective viewing duration of the document.
13. The computer-readable medium of claim 11 wherein the first number is a weighted combination of a plurality of the views of the document wherein one or more of the views is weighted using a respective weight based at least in part on a respective viewing duration of the document. 16. The computer-readable medium of claim 13 wherein the weight is determined using a discontinuous function of viewing length.