about user/item scaling

[Claudio Martella <cl...@tis.bz.it>]

Hi,


I'm gettin into mahout through the reading of Mahout in action. I'm
pretty curious about this sentence:

[talking about item-based an user-based reccomendation systems]

"They do have notably different properties. For instance, the running
time of an item-based
recommender scales up as the number of items increases, whereas a
user-based recommender’s
running time goes up as the number of users increases.
This suggests one reason that you might choose an item-based
recommender: if the number of users
is relatively low compared to the number of items, the performance
advantage could be significant."

How would the item-based recommender be a good choice with a huge number
of items if the item-based recommender's cost grows with the number of
items?


thanks


-- 
Claudio Martella
Digital Technologies
Unit Research & Development - Analyst

TIS innovation park
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Tel. +39 0471 068 123
Fax  +39 0471 068 129
claudio.martella@tis.bz.it http://www.tis.bz.it

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Re: about user/item scaling

[Ted Dunning <te...@gmail.com>]

The cost of an item-based recommender can depend a lot on the
implementation.  If you make the access to all of the related items of a
single item be O(1), then the cost of item-based recommendations is only
proportional to the size of the user history that you consider.

Basically what happens here is that much of the work of the user based
recommendation system has been move to an off-line process.  This very much
limits your update rate, but related items don't change terribly quickly.

On Sat, Feb 27, 2010 at 10:28 AM, Claudio Martella <
claudio.martella@tis.bz.it> wrote:

> "They do have notably different properties. For instance, the running
> time of an item-based
> recommender scales up as the number of items increases, whereas a
> user-based recommender’s
> running time goes up as the number of users increases.
>

-- 
Ted Dunning, CTO
DeepDyve

Re: about user/item scaling

[Sean Owen <sr...@gmail.com>]

That's a pure typo. I will fix that.

On Sat, Feb 27, 2010 at 6:28 PM, Claudio Martella
<cl...@tis.bz.it> wrote:
> Hi,
>
>
> I'm gettin into mahout through the reading of Mahout in action. I'm
> pretty curious about this sentence:
>
> [talking about item-based an user-based reccomendation systems]
>
> "They do have notably different properties. For instance, the running
> time of an item-based
> recommender scales up as the number of items increases, whereas a
> user-based recommender’s
> running time goes up as the number of users increases.
> This suggests one reason that you might choose an item-based
> recommender: if the number of users
> is relatively low compared to the number of items, the performance
> advantage could be significant."
>
> How would the item-based recommender be a good choice with a huge number
> of items if the item-based recommender's cost grows with the number of
> items?
>
>
> thanks
>
>