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Date: Tue, 23 Nov 2010 07:50:09 +0000
Message-ID: <AANLkTimsP-nkS_0vVLKCMqBOYGzdfvEWkUUG9LfDRFBH@mail.gmail.com>
Subject: Re: Matrix-based recommendation analysis
From: Sean Owen <srowen@gmail.com>
To: user@mahout.apache.org
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(PS I don't think that link from Ted is publicly visible but try
http://www.slideshare.net/tdunning )

Maybe I'm walking into half of a another conversation but what's the
question or goal here?

I don't think the matrix product contains quite what you're saying.
For example U1 records only 2 ratings but has some "enthusiasm" on 3
separate days in the matrix product. The product is mashing together
item-day associations from all users and applying them to each user.

Conceptually user-item-day is the 3-dimensional matrix that it sounds
like, if you want to distinguish associations from different users to
different items on different days.


On Tue, Nov 23, 2010 at 7:24 AM, Lance Norskog <goksron@gmail.com> wrote:
> The GroupLens dataset has User, Item, Rating and Timestamp.
> We will use the rating of 1-5 as-is, but will reduce the timestamp
> field to day of the week.
> The lack of a rating defaults two 3 (neutral). There are 5 ratings
> total in the sample:
>
> U1, I1, 2, ?
> U1, I3, 4, ?
> U2, I1, 4, ?
> U2, I2, 5, T
> U2, I3, 3, ?
>
> (We'll get to the question marks later.)
> Now, make two matrices, User v.s. Item and Item v.s. Day of the Week.
> User v.s. Item contains ratings, and Item v.s. Day of the Week
> contains the number of rating records for that item on that day of the
> week: ratings only cover Sunday, Monday and Tuesday.
>
> Formatting tables in kerned fonts just plain doesn't work, thus the
> alternate format.
>
> 2 Users v.s. 3 Items:
> I1,I2,I3
> {
> U1 =C2=A0{2,3,4}
> U2 =C2=A0{4,5,3}
> =C2=A0}
>
> 3 Items v.s. 7 Days of the Week
> S,M,T,W,T,F,S
> {
> I1 {1,0,1,0,0,0,0}
> I2 {0,0,1,0,0,0,0}
> I3 {0,1,1,0,0,0,0}
> }
>
> Now, multiply these two matrices. The product is 2 Users v.s. 7 Days
> of the Week:
> S,M,T,W,T,F,S
> {
> U1 {2,4,9,0,0,0,0}
> U2 {4,3,12,0,0,0,0}
> }
>
> This matrix carries the total amount of enthusiasm for each user on
> each day. To get the average enthusiasm of each user, divide each row
> by the total number of ratings per day:
> S,M,T,W,T,F,S
> {
> U1 {2,4,3,0,0,0,0}
> U2 {4,3,4,0,0,0,0}
> }
>
> Did I get this right, Ted?
>
> BTW, where are your slides for this topic? I've seen them a couple of
> times in presentations (live and on Fora.tv), but can't find them.
>
> --
> Lance Norskog
> lance.norskog@gmail.com
>