A HYBRID GA-BASED COLLABORATIVE FILTERING MODEL
FOR ONLINE RECOMMENDERS
Yvonne Ho, Simon Fong and Zhuang Yan
Faculty of Science and Technology, University of Macau
Keywords: Collaborative Filtering, Recommendation Systems, Genetic Algorithm.
Abstract: Online recommenders have been a prevalent tool in e-Business that assists users to find items of their
interest. Different algorithms that predict scores based on the heuristic of similarity of other peoples’ taste to
that of the user have been experimented by many researchers. However most of the current recommendation
systems take all users with common items as neighbors in their measurement, which may induce noise and
hence inaccurate prediction. In this paper, we attempt to solve this problem by proposing a hybrid model
that combines content-based filtering and collaborative filtering for online recommenders. The model
exploits merits from these two techniques by selectively encoding both the user profiles and the product
information into the same chromosomes in a Genetic Algorithm. Our experiments demonstrated that this
new approach gives relatively high accuracy rate in predicting user preferences.
1 INTRODUCTION
Generally, there are two common methods applied
in recommendation systems for predicting the users’
items of interest, they are Collaborative Filtering,
and Content-based Filtering. Collaborative Filtering
(CF) method (Breese, Heckerman, and Kadie, 1998)
is based on the similarity between currently active
user and other users. It can either be measured by the
same item which is known as item-based CF or by
the same type of user, known as user-based CF. The
goal of this method is to suggest new items, to
predict the utility of a certain item for a particular
user based on his previous likings or the opinions of
other like-minded users. In the traditional CF
method, the system only searches for users who
contain common items with the active user. In this
case, only the top popular items will be selected. As
a few common items are insufficient to reflect a
user’s taste, the recommender will not be very
accurate.
Content-based Filtering selects items based on
the correlation between the content of the items and
user preferences. It recommends the items similar to
those a given user has liked in the past. There are
however some short-comings while applying
Content-based Filtering technique on recommenders.
Firstly, for text documents, the system can only
capture certain aspects of the content, so that only a
shallow analysis of certain kinds of content can be
supplied. Secondly, the system can only recommend
items scoring highly against the user profile, so the
user is restricted to see the items similar to those
already rated, new items will seldom be
recommended. Furthermore, the user’s own rating is
the only factor influencing the future performance.
A Hybrid model combines the Collaborative
Filtering and Content Based Method. However, a
mere integration of the two techniques suffers from
prohibited oversize of data and a very long
processing time. In our proposed model, a genetic
algorithm (GA) in such hybrid CF was used in order
to give a reasonably quick and optimally accurate
prediction, while avoiding the need of large amount
of data from both users and items information.
One key feature of this hybrid model powered by
GA is feature (and feature weights) selection. This is
to eliminate the situation where redundant features
were included when making a prediction. They are
noises that damper the prediction accuracy of the
recommendation during calculation.
In this paper, we described a GA-based (Ujjin
and Bentley, 2002) hybrid CF model and
experimented this model on an online movie
recommendation system for performance evaluation.
The model is able to select the appropriate feature
weights in the computation.
200
Ho Y., Fong S. and Yan Z. (2007).
A HYBRID GA-BASED COLLABORATIVE FILTERING MODEL FOR ONLINE RECOMMENDERS.
In Proceedings of the Second International Conference on e-Business, pages 200-203
DOI: 10.5220/0002113602000203
Copyright
c
 SciTePress

Figure 1: A typical workflow of CF online recommender that includes user’s profiles.
Profile Capture:
User registered to provide
the demographics
New user
Movie
Database
Collect Movie Info
Recommendations
Data
Collect selected items
from neighbors
Search for neighbors
from user database
User
Database
Neighbor Set
GA
Recommender
Get Top N
neighbors
Collect Ratings:
Get ratings of movies
Create New User Profile
Features
Phase I Phase II Phase III: Recommender
Profile Capture:
User registered to provide
the demographics
New user
Movie
Database
Movie
Database
Collect Movie Info
Recommendations
Data
Collect selected items
from neighbors
Search for neighbors
from user database
User
Database
User
Database
Neighbor Set
GA
Recommender
Get Top N
neighbors
Collect Ratings:
Get ratings of movies
Create New User Profile
Features
Phase I Phase II Phase III: Recommender
Figure 2: GA-based Recommendation System.
In this example case, we showed that user profile
features are most important to the target user in
relative to other movie attributes which may turn
into noises. Moreover, the similarity measure can
collect the neighbor sets with a similar taste; in this
way the recommendations will be more adaptable.
From the results of our experiment, we show that
GA combined with some fine-tuned User Profiles
features is a good candidate for recommendation
systems.
2 GA RECOMMENDER MODEL
Based on the techniques mentioned in the previous
section, we constructed a GA-based Recommender
as a prototype model for conducting experiments.
The experimental protocol is capable of
gathering, disseminating, and using ratings from
some users to predict other users' interest in movies.
A general workflow of an online recommender
model is shown in Figure 1. Comparing to GA,
Pearson Algorithm is fast though the prediction is
not as accurate. An e-Business website may consider
using Pearson recommender for instant notification
of recommended results. If the user however desires
recommended results of higher accuracy, GA which
usually takes a few minutes for processing can send
the results by email. This process can be done
offline, perhaps in batches. Figure 2 shows the
architecture of our system that can be deployed as an
online application. The process flow is divided into
three phases:
• Phase I: Collect User Information
• Phase II: Create User Profile features
• Phase III: GA Recommender
This phase has the following GA related
functions that search for the appropriate
A HYBRID GA-BASED COLLABORATIVE FILTERING MODEL FOR ONLINE RECOMMENDERS
201

recommendation by selecting and weighing the
features.
2.1 Feature Selection
The first step in the GA Recommender phase is to
prepare the features that are needed. From the
experiments we observed that some of the user
profile attributes are more effective than the others.
So we only select 12 features out of 37: Rating, Age,
Gender, Occupation, Prefer FilmType, Prefer
Director, Prefer Actress, Prefer Actor, Prefer
Producer, Prefer Writer, Prefer Editor and Prefer
Language.
2.2 Feature Weighing
The structure of the chromosomes is similar to
Figure 1. There are totally 12 genes. Each gene
represents by a feature weight w in real value. The
heavier the weight the more important the feature is,
so that the value can represent the feature
importance to the user. For example, the weight of
the feature Prefer Director is the highest, that
indicates the user favors over his choice movies by
certain movie directors.
We programmed in GALib (GALib) to find out
the feature weight w, distance measure d and obtain
a group of neighbor set for the active user by
choosing half of the top scores from the users of
similar taste.
2.3 Recommendations
After we obtained the neighbor set, we can provide
the active user a list of recommendations by
summarizing the neighbor’s rated movies which
have not been rated by the active user. Also we can
predict the votes of those movies, that helps guiding
the user to choose his favorites by ranking the votes.
The vote for movie i for active user a can be
predicted by:
∑
=
−+=
n
j
jija vvjaeuclideankviavotepredict
1
))(,(),(_ ,
As we can also measure the predict vote for those
movies that the active user rated before, we can
compare it with the actual vote to cross-check the
fitness rate for user a on movie i:
),(_
%100),(_
),(
iavoteactual
iavotepredict
iafitness
×
=
3 EXPERIMENTS
In order to evaluate the effectiveness of our work,
we would compare the traditional Collaborative
Filtering method using Pearson Coefficient and our
proposed schemes by using Genetic Algorithm. We
repeated the experiments by using different features
as to show how the feature weights affect the fitness
accuracy and performance.
3.1 Experiment I - Features Weights
We calculated the weights for each feature, and took
ten users to test the importance of features to each
user. We show the average weights for each feature
on different users. The average feature weights is
about 0.2649.
The separation of two groups of line is evident
that the weights of user profile features are much
higher than other movie attributes. Especially the
features prefer director, prefer actress, prefer actor,
prefer producer, prefer writer, prefer editor, all of
their feature weights are over 0.3.
By this observation, we assume that rating, age,
gender, occupation, prefer film type, prefer director,
prefer actress, prefer actor, prefer producer, prefer
writer, prefer editor are more relevant to the user
preference.
3.2 Experiment 2 - Fitness Accuracy
For testing the fitness accuracy of Pearson
Algorithm and Genetic Algorithm, and also the
effectiveness of different features on GA, we
configured a variety of six components in this
experiment and applied them on 50 fixed users
Table 1: Compositions of chromosome types.
Chromosome Features Chromosome Type
1 movie rating and 3
user’s particulars
8 user profile
features
18 movie
genres
7 movie
characters
Pearson Algorithm (Pearson)
9 9 9
Genetic Algorithm with 22 features (GA)
9 9
Genetic Algorithm with 29 features (GA Merge)
9 9 9
Genetic Algorithm with 37 features (GA User Profile 37)
9 9 9 9
Genetic Algorithm with 12 features (GA User Profile 12)
9 9
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202

respectively. The chromosome compositions of the
five components in terms of features are shown in
Table 1.
The last component is constructed by
preprocessing the user profile data pertaining to the
movie attributes, that has a set of 7 unique features.
We can observe apparently that prediction of GA
is much higher than that of Pearson. The average
fitness of Pearson is about 67.9%, whereas the
average fitness for ‘GA’ with various combinations
of features range from 83.51% to 83.93%. In
particular, the fitness of ‘GA UserPro12’ is the
highest, about 23.61% better than that of Pearson.
The experiment shows the process time for
running GA on different features. ‘GA UserPro7’
that is GA with 7 features out performs the other 4 in
terms of speed. Its average process time is 19
seconds. This is a 67.53% reduction over ‘GA’.
From the experiment, the longest time taken is
by ‘GA UserPro37’ and the shortest time is ‘GA
UserPro7’. This reinforces the belief that when more
features are into the recommender a longer
processing time it takes.
3.3 Experiment 3 - Neighbor Set
For a particular user, we tested the performance on
different group sizes of neighbor set, from 10 to 100
respectively.
3.3.1 Process Time vs. Neighbor Set
This experiment shows the performance of different
features running on GA with different neighbor sets.
At the beginning, their performances are close. As
the neighbor set size increases, the process time for
‘GA’, ‘GA Merge’, ‘GA UserPro37’ increase quite
sharply. The additional features they have in
common are the 18 movie genres.
As we can see, ‘GA UserPro37’ for 37 features,
the process time increases gradually as the neighbor
set expands; where for ‘GA UserPro7’ with 7
features, the process time increases slowly.
3.3.2 Fitness vs. Neighbor Set
In the experiment, the fitness of different features
rise up gradually as the neighbor set enlarges.
Interestingly when the neighbor set reaches over the
size of 35, the fitness continues to stay constant. As
indicated by the dotted line in chart, the fitness
approaches 95% at the turning point. Further
increase on the neighbor set size has no effect on it.
4 CONCLUSION
Our proposed GA-based hybrid CF model combines
both correlation analysis of active user to users, and
contents of the items rated by peer users. The
information of user profiles and item attributes are
encoded accordingly into GA chromosomes. As our
experiments show, this GA model offers more
accurate recommendation than that of Pearson
Algorithm. By applying user profile features that are
more valuable than other features such as movie
genres on GA, the similarity measure finds the
neighbors with similar taste to the user; as a result,
the user preference can be better predicted. And
when user profile features are used alone, the
process speeds up. In essence, this hybrid approach
exploits merits from CF techniques by selectively
encoding both the user profiles and the product
information into the same chromosomes in a Genetic
Algorithm. Our experiment also shows that the GA
fitness keeps constant when neighbor set size
increases. This implies some positive elements in the
scalability and speed issues of GA online
recommendation systems.
REFERENCES
Breese, J., Heckerman, D., and Kadie, C., 1998.Empirical
Analysis of Predictive Algorithms for Collaborative
Filtering. Proceedings of the 14th Conference on
Uncertainty in Artificial Intelligence.
Ujjin, S., Bentley, P., 2002. Evolving Good
Recommendations. Genetic and Evolutionary
Computation Conference (GECCO).
GALib, http://lancet.mit.edu/ga/
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