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From Jonathan Hodges <>
Subject Re: Can someone suggest an approach for calculating precision and recall for distributed recommendations?
Date Mon, 27 Aug 2012 00:17:29 GMT
Thanks for your thorough response.  It is really helpful as we are new to
Mahout and recommendations in general.  The approach you mention about
training on data up to a certain point a time and having the recommender
score the next actual observations is very interesting.  This would seem to
work well with our Boolean dataset.  We will give this a try.

Thanks again for the help.


On Sun, Aug 26, 2012 at 3:55 PM, Sean Owen <> wrote:

> Most watched by that particular user.
> The issue is that the recommender is trying to answer, "of all items
> the user has not interacted with, which is the user most likely to
> interact with"? So the 'right answers' to the quiz it gets ought to be
> answers to this question. That is why the test data ought to be what
> appears to be the most interacted / preferred items.
> For example If you watched 10 Star Trek episodes, then 1 episode of
> the Simpsons, and then held out the Simpson episode -- the recommender
> is almost surely not going to predict it, not above more Star Trek.
> That seems like correct behavior, but would be scored badly by a
> simple precision test.
> There are two downsides to this approach. Firstly removing well liked
> items from the training set may meaningfully skew a user's
> recommendations. It's not such a big issue if the test set is small --
> and it should be.
> The second is that by taking out data this way you end up with a
> training set which never really existed at one point in time. That
> also could be a source of bias.
> Using recent data points tends to avoid both of these problem -- but
> then has the problem above.
> There's another approach I've been playing with, which works when the
> recommender produces some score for each rec, not just a ranked list.
> You can train on data up to a certain point in time, then have the
> recommender score the observations that really happened after that
> point. Ideally it should produce a high score for things that really
> were observed next.
> This isn't implemented in Mahout but you do get a score with recs even
> without ratings.

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