23.6.1.1 Output Estimates Change (Y-Change)
This approach [42] operates on the principle that if rating estimates do not change
then they will not improve. Thus, if the estimates of output values do change, then
their accuracy may either increase or decrease. However, it is expected that at least
something will be learned from a new training point, so it follows then that in many
cases estimates do in fact become more accurate. Assuming that most changes in
estimates are for the better, an item that causes many estimates to change will result
in the improvement of many estimates, and is considered useful.
As an example (Figure 23.6), if a user rates an item that is representative of a
large genre, such as the Sci-Fi movie Star Wars, then its rating (regardless of its
value) will likely cause a change in rating estimates for many other related items
(e.g. items within that genre), in other words, rating such a representative item is
very informative about the user’s preferences. On the other hand, the user rating
an item without many other similar items, such as the movie The Goldfish Hunter,
would change few rating estimates, and supply little information.
To find the expected changes in rating estimates caused by a candidate item’s
rating, all possible item ratings are considered (since the true rating of a candidate
item is not yet known). The difference is calculated between rating estimates for
each item for each of its possible ratings, before and after it was added to the training
set (refer to the pseudocode in Algorithm 1).
More formally the above criterion could be expressed as: