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A Recommender system based on the Immune Network Proceedings CEC2002, pp 807-813, Honolulu, USA, 2002 Steve Cayzer and Uwe Aickelin Hewlett-packardlaBoratoriesFiltonRoadBs126qzBristol,Uk,stevecayzer@hp.com School of Computer Science, University of Nottingham, NG8 1BB UK, uxa @cs. nott. ac uk Abstract-The immune system is a complex biological system continue to spread through the population and potentially has with a highly distributed, adaptive and self-organising nature. much explanatory power. The idiotypic network has been This paper presents an artificial immune system(AIS)that formalised by a number of theoretical immunologists[15] exploits some of these characteristics and is applied to the task There are many more features of the immune system, evolution and in particular the immune system have not been including adaptation, immunological memory and protection designed for classical optimisation. However, for this problem, against auto-Immune attack. Since these are not directly we are not interested in finding a single optimum. Rather we relevant to this work, they will not be reviewed here ntend to identify a sub-set of good matches on which recommendations can be based. It is our hypothesis that an AIs Overview of collaborative Filtering be an ideal candidate to achieve this: Antigen .antibody t In this paper, we are using an AIS as a CF technique. CFis term for a broad range of algorithms that use similarity interaction for matching and antibody - antibody interaction for measures to obtain recommendations. The best-known diversity.computatioNalresultsarepresentedinsupportofthisexampleisprobablythe"peoplewhoboughtthisalso bought"feature of the internet company Amazon [2] However, any problem domain where users are required to . INTRODUCTION rate items is amenable to CF techniques. Commercial applications are usually called recommender systems [16].A Over the last few years, a novel computational intelligence canonical example is movie recommendation chnique, inspired by biology, has emerged: the artificial In traditional CF, the items to be recommended are treated immune system(AlS). This section introduces the AIs and That is, yo commendations are based problems. In essence, the immune system is used here as content of the item. The preferences of a user, usually a set of inspiration to create an unsupervised machine-learning votes on an item, comprise a user profile, and these profiles lgorithm. The immune system metaphor will be explore are compared to build a neighbourhood. The key decisions to involving a brief overview of the basic immunological be made are theories that are relevant to our work. We also introduce the Data encoding: Perhaps the most obvious representation basic concepts of collaborative filtering(CF) for a user profile is a string of numbers, where the length the number of items, and the position is the item identifier Overview of the Immune System Each number represents the 'vote for an item. Votes are a detailed overview of the immune system can be found sometimes binary(e.g. did you visit this web page? but can many textbooks [14]. Briefly, the purpose of the immune also be integers in a range(say [0, 5])or rational numbers stem is to protect the body against infection and includes a Similarity Measure: The most common method to cor set of mechanisms collectively termed humoral immunity. two users is a correlation-based measure like Pearson or This refers to a population of circulating white blood cells Spearman, which gives two neighbours a matching score called B-lymphocytes, and the antibodies they create The features that are particularly relevant to our research between vectors, and probabilistic methods are alternative are matching, diversity and distributed control. Matching The canonical example is the k Nearest Neighbo Diversity refers to the fact that, in order to achieve optimal algorithm, which uses a matching method to select k antigen space coverage, antibody diversity must be reviewers with high similarity measures. The votes from hese reviewers, suitably weighted, are used to make encouraged [11]. Distributed control means that there is no predictions and recommendations central controller, rather, the immune system is governed by local interactions between cells and antibodies Many improvements on this method are possible [10]. For The idiotypic network hypothesis [13](disputed by some example, the user profiles are usually extremely sparse immunologists )builds on the recognition that antibodies can because many items are not rated. This means that similarity match other antibodies as well as antigens. Hence,an antibody may be matched by other antibodies, which in turn dimensionality,) and difficult to calculate due to the small may be matched by yet other antibodies. This activation can overlap. Default votes are sometimes used for items a user

A Recommender System based on the Immune Network Proceedings CEC2002, pp 807-813, Honolulu, USA, 2002. Steve Cayzer1 and Uwe Aickelin2 1Hewlett-Packard Laboratories, Filton Road, BS12 6QZ Bristol, UK, steve_cayzer@hp.com 2 School of Computer Science, University of Nottingham, NG8 1BB UK, uxa@cs.nott.ac.uk Abstract-The immune system is a complex biological system with a highly distributed, adaptive and self-organising nature. This paper presents an artificial immune system (AIS) that exploits some of these characteristics and is applied to the task of film recommendation by collaborative filtering (CF). Natural evolution and in particular the immune system have not been designed for classical optimisation. However, for this problem, we are not interested in finding a single optimum. Rather we intend to identify a sub-set of good matches on which recommendations can be based. It is our hypothesis that an AIS built on two central aspects of the biological immune system will be an ideal candidate to achieve this: Antigen - antibody interaction for matching and antibody - antibody interaction for diversity. Computational results are presented in support of this conjecture and compared to those found by other CF techniques. I. INTRODUCTION Over the last few years, a novel computational intelligence technique, inspired by biology, has emerged: the artificial immune system (AIS). This section introduces the AIS and shows how it can be used for solving computational problems. In essence, the immune system is used here as inspiration to create an unsupervised machine-learning algorithm. The immune system metaphor will be explored, involving a brief overview of the basic immunological theories that are relevant to our work. We also introduce the basic concepts of collaborative filtering (CF). Overview of the Immune System A detailed overview of the immune system can be found in many textbooks [14]. Briefly, the purpose of the immune system is to protect the body against infection and includes a set of mechanisms collectively termed humoral immunity. This refers to a population of circulating white blood cells called B-lymphocytes, and the antibodies they create. The features that are particularly relevant to our research are matching, diversity and distributed control. Matching refers to the binding between antibodies and antigens. Diversity refers to the fact that, in order to achieve optimal antigen space coverage, antibody diversity must be encouraged [11]. Distributed control means that there is no central controller, rather, the immune system is governed by local interactions between cells and antibodies. The idiotypic network hypothesis [13] (disputed by some immunologists) builds on the recognition that antibodies can match other antibodies as well as antigens. Hence, an antibody may be matched by other antibodies, which in turn may be matched by yet other antibodies. This activation can continue to spread through the population and potentially has much explanatory power. The idiotypic network has been formalised by a number of theoretical immunologists [15]. There are many more features of the immune system, including adaptation, immunological memory and protection against auto-immune attack. Since these are not directly relevant to this work, they will not be reviewed here. Overview of Collaborative Filtering In this paper, we are using an AIS as a CF technique. CF is the term for a broad range of algorithms that use similarity measures to obtain recommendations. The best-known example is probably the “people who bought this also bought” feature of the internet company Amazon [2]. However, any problem domain where users are required to rate items is amenable to CF techniques. Commercial applications are usually called recommender systems [16]. A canonical example is movie recommendation. In traditional CF, the items to be recommended are treated as ‘black boxes’. That is, your recommendations are based purely on the votes of your neighbours, and not on the content of the item. The preferences of a user, usually a set of votes on an item, comprise a user profile, and these profiles are compared to build a neighbourhood. The key decisions to be made are: Data encoding: Perhaps the most obvious representation for a user profile is a string of numbers, where the length is the number of items, and the position is the item identifier. Each number represents the 'vote' for an item. Votes are sometimes binary (e.g. did you visit this web page?) but can also be integers in a range (say [0,5]) or rational numbers. Similarity Measure: The most common method to compare two users is a correlation-based measure like Pearson or Spearman, which gives two neighbours a matching score between -1 and 1. Vector based, e.g. cosine of the angle between vectors, and probabilistic methods are alternative approaches. The canonical example is the k Nearest Neighbour algorithm, which uses a matching method to select k reviewers with high similarity measures. The votes from these reviewers, suitably weighted, are used to make predictions and recommendations. Many improvements on this method are possible [10]. For example, the user profiles are usually extremely sparse because many items are not rated. This means that similarity measurements are both inefficient (the so-called ‘curse of dimensionality’) and difficult to calculate due to the small overlap. Default votes are sometimes used for items a user

has not explicitly voted on, and these can increase the overlap identify a suitable neighbourhood. The SwAMI(Shared size [4]. Dimensionality reduction methods, such as Single Wisdom through the Amalgamation of Many Interpretations) Value Decomposition, both improve efficiency and increase framework [9 is a publicly accessible software for CF overlap 3. Other pre-processing methods are often used, e. g. experiments. Its central algorithm is as follows clustering [1]. Content-based information can be used to enhance the pure CF approach [10], [6]. Finally, the Select a set of test users randomly from the database weighting of each neighbour can be adjusted by training FOR each test user t there are many learning algorithms available for this [7 All Reserve a vote of this user, i.e. hide from predictor) these improvements could in principle be applied to our Als Select neighbourhood of k reviewers based onr.a. From remaining votes create a new training use but in the interests of a clear and uncluttered comparison we The evaluation of a CF algorithm usually centres on its NEr// b5bourhood to predict vote have kept the CF algorithm as simple as possible Use ne Com s with actual vote and collect statistics accuracy. There is a difference between prediction(given a movie,predict a given user's rating of that movie)and The code shown in italics indicates a place where SWAMI recommendation(given a user, suggest movies that are likely allows an implementation-dependent choice of algorithm W to attract a high rating). Prediction is easier to assess use an AIS to perform selection and prediction as below quantitatively but recommendation is a more natural fit to the movie domain. We present results evaluating both these Algorithm Choices behaviou We use the Swami data encoding Using an AlS for Collaborative Filtering User=flid, score, ) id,, score, )idm, score, B To us, the attraction of the immune system is this: if an Where id corresponds to the unique identifier of the movie adaptive pool of antibodies can produce intelligent being rated and score to this user's score for that movie. This behaviour, can we harness the power of this computation to captures the essential features of the data available EachMovie vote data links a per a movie and antibodies and the new preferences to be matched is the age and gender) is provided but this is not used in our antigen in question. encoding. Content information about movies(e.g. category) Our conjecture is that if the concentrations of those is similarly not used. ntibodies that provide a better match are allowed to increase over time, we should end up with a subset of good matches. Similarity Measure However, we are not interested in optimising, 1.e. in finding The Pearson measure is used to compare two users u and v the one best match. Instead, we require a set of antibodies that are a close match but which at the same time distinct ∑(u-n)v,-) from each other for successful recommendation. This is p where we propose to harness the idiotypic effects of binding antibodies to similar antibodies to encourage diversity. u1-)∑(m-) The next section presents more details of our problem explains the AIs model we intend to use. We then describe Where u and v are users, n is the number of overlapping the experimental set-up and present some initial results. votes(i. e. movies for which both u and v have voted), u,is Finally we review the results and discuss some possibilities the vote of user u for movie i and ii is the average vote of for future work user u over all films(not just the overlapping votes).The measure is amended as follows 2. ALGORITHMS if n=0, r= NoOverlapDefault Application of the Als to the EachMovie Tasks j∑u-a)∑(n,-)=0.,r= Zero variance Default(2) The EachMovie database [5] is a public database, which i= votes taken from 72,916 users on 1,628 films. The task is to n<, r=n (where P=overlap penalty records explicit votes of users for movies. It holds 2, 811,983 use this data to make predictions and recommendations. In the former case, we provide an estimated vote for a The two default values are required because it is previously unseen movie. In the latter case, we present a impossible to calculate a Pearson measure in such cases.Both ranked list of movies that the user might like were set to O Some experimentation showed that an overlap The basic approach of CF, is to use information from a penalty P was beneficial(this lowers the absolute correlation neighbourhood make useful predictions and for users with only a small overlap) but that the exact value recommendations. The central task we set ourselves is to

was not critical. We choose a value of 100 because this is the The Als is considered stable after iterating for ten maximum overlap expected iterations without changing in size. Stabilisation thus means hat a sufficient number of good neighbours have been Neighbourhood Selection identified and therefore a prediction can be made. Poor For a Simple Pearson predictor, neighbourhood selection neighbours would be expected to drop out of the Als after a means simply choosing the best k(absolute) correlation few iterations scores,where k is the neighbourhood size. Not every Once the als has stabilised using the above algorithm, w potential neighbour will have rated the film to be predicted. use the antibody concentration to weigh the neighbours Reviewers who did not vote on the film are not added to the However, early experiments showed that the most recently neighbourhood dded antibodies were at a disadvantage compared to earlier antibodies. This is because they have had no time to mature For the Als predictor, a more involved procedure is required: (i.e. increase in concentration). Likewise, the earliest antibodies had saturated. To overcome this we reset the Initialise AIs concentrations and allow a limited run of the ais to Encode user for whom to make predictions as ar differentiate the concentrations. WHILE(AIS not stabilised )&( Reviewers avai Add next user as an antibody ab Reset al Calculate matching scores between Ab and Ag all antibodies to initial concentrations) WHILE Calculate matching scores between Ab and other antibodies ntibody at maximum concentration) DO WHILE (AIS at full size)&(AlS not stable)DO OD Iterate AIs OD Prediction We predict a rating P, by using a weighted average over N, Dur AIS behaves as follows: At each step (iteration)an the neighbourhood of u, which was taken as the entire AIS antibodys concentration is increased by an amount amount which depends on its matching to other antibodies In p,=i+R n(v-F) y (4) bsence of either, an antibodys concentration will slowly decrease over time. Antibodies with a sufficiently low concentration are removed from the system, whereas wom=Ia, r, (NB relative not absolute) antibodies with a high concentration may saturate. An AIs iteration is governed by the following equation: Where Wu is the weight between users u and 1, ran is the correlation score between u and v, and xr is the concentration tibody death of the antibody corresponding to user v. dh stimulation)(su pression)(rate allanton kmxy-∑mxx一kx (3) Prediction Accuracy: We take the mean absolute error number of pre k ,= Stimulation, ka k, Death rate ∑ actual- predicted N= Number antibodies MAE= (5) x or y)=concentration of antibody (or antigen) Mean number of recommendations: This is the tota This is a slightly modified version of Farmer Is number of unique films rated by the neighbours quation [ 8]. In particular, the first term is simplified as we only have one antigen, and we normalise the suppression Mean overlap size: This is the number of recommendations term to allow a 'like for like'comparison between the that the user has also seen different rate constants. k, and k, were varied as described in the next section. k, was fixed at 0. 1, while the concentration Mean accuracy of recommendations: Each overlapped film range was set at 0-100 (initially 10). We fixed N at 100. The has an actual vote(from the antigen) and a predicted vote matching function is the absolute value of the Pearson (from the neighbours). The overlapped films were ranked or correlation measure. This allows us to have both positively both actual and predicted vote, breaking ties by movie ID and negatively correlated users in our neighbourhood, which The two ranked lists were compared using Kendalls Tau t increases the pool of neighbours available to us This measure reflects the level of concordance in the lists by

was not critical. We choose a value of 100 because this is the maximum overlap expected. Neighbourhood Selection For a Simple Pearson predictor, neighbourhood selection means simply choosing the best k (absolute) correlation scores, where k is the neighbourhood size. Not every potential neighbour will have rated the film to be predicted. Reviewers who did not vote on the film are not added to the neighbourhood. For the AIS predictor, a more involved procedure is required: Initialise AIS Encode user for whom to make predictions as antigen Ag WHILE (AIS not stabilised) & (Reviewers available) DO Add next user as an antibody Ab Calculate matching scores between Ab and Ag Calculate matching scores between Ab and other antibodies WHILE (AIS at full size) & (AIS not stable) DO Iterate AIS OD OD Our AIS behaves as follows: At each step (iteration) an antibody’s concentration is increased by an amount dependent on its matching to the antigen and decreased by an amount which depends on its matching to other antibodies. In absence of either, an antibody’s concentration will slowly decrease over time. Antibodies with a sufficiently low concentration are removed from the system, whereas antibodies with a high concentration may saturate. An AIS iteration is governed by the following equation: ( ) ( ) , , )3( 1 2 3 3 1 2 1 x or y concentration of antibody or antigen N Number antibodies k Stimulation k Suppression k Death Rate m r m x x k x N k k m x y rate death su ppression antibody stimulation antigen dt dx i ij i N j i i ij i j i = = = = = = = − −         −        −        = ∑− This is a slightly modified version of Farmer et al’s equation [8]. In particular, the first term is simplified as we only have one antigen, and we normalise the suppression term to allow a ‘like for like’ comparison between the different rate constants. k1 and k2 were varied as described in the next section. k3 was fixed at 0.1, while the concentration range was set at 0–100 (initially 10). We fixed N at 100. The matching function is the absolute value of the Pearson correlation measure. This allows us to have both positively and negatively correlated users in our neighbourhood, which increases the pool of neighbours available to us. The AIS is considered stable after iterating for ten iterations without changing in size. Stabilisation thus means that a sufficient number of ‘good’ neighbours have been identified and therefore a prediction can be made. ‘Poor’ neighbours would be expected to drop out of the AIS after a few iterations. Once the AIS has stabilised using the above algorithm, we use the antibody concentration to weigh the neighbours. However, early experiments showed that the most recently added antibodies were at a disadvantage compared to earlier antibodies. This is because they have had no time to mature (i.e. increase in concentration). Likewise, the earliest antibodies had saturated. To overcome this, we reset the concentrations and allow a limited run of the AIS to differentiate the concentrations: Reset AIS (set all antibodies to initial concentrations) WHILE (No antibody at maximum concentration) DO Iterate AIS OD Prediction We predict a rating pi by using a weighted average over N, the neighbourhood of u, which was taken as the entire AIS. ( ) ( ) )4( w r x NB relative not absolute w w v v p u uv uv v v N uv v N uv i i = − = + ∑ ∑ ∈ ∈ Where wuv is the weight between users u and v, ruv is the correlation score between u and v, and xv is the concentration of the antibody corresponding to user v. Evaluation Prediction Accuracy: We take the mean absolute error, where np is the number of predictions: )5( np actual predicted MAE ∑ − = Mean number of recommendations: This is the total number of unique films rated by the neighbours. Mean overlap size: This is the number of recommendations that the user has also seen. Mean accuracy of recommendations: Each overlapped film has an actual vote (from the antigen) and a predicted vote (from the neighbours). The overlapped films were ranked on both actual and predicted vote, breaking ties by movie ID. The two ranked lists were compared using Kendall’s Tau τ. This measure reflects the level of concordance in the lists by

counting the number of discordant pairs. To do this we order tion on number of users looked at the films by vote and apply the following formulae N=∑∑D(,) D(,r) ifr> otherwise Figure 1: Effect of stimulation rate on neighbourhood and reviewers Where n is the overlap size and r, is the rank of film i as ecommended by the neighbourhood. Note that i here refers The graphs show averaged results over five runs at each to the antigen rank of the film, not the film ID. No is the stimulation rate. The bars show standard deviations. In order to have a fair comparison, the Simple Pearson parameters cost of a bubble sort to reconcile the two lists. d is set to one (neighbourhood and number of reviewers looked at)match if the rankings are discordant the Ais values for each rate. In figure 2, we show the prediction error, number of recommendations, number of Mean number of reviewers. This is the num ber overlaps and recommendation accuracy for each algorithm reviewers looked at before the ais stabilised Note that low prediction error values are better, whereas for the other measures we are looking for high values Mean number of neighbours This is the final number of neighbours in the stabilised AIS 3 EXPERIMENTS Experiments were carried out on a Pentium 700 in JavaTM JDK13 Each run involved looking at up to 15,000 as/ 256MB RAM, running Windows 2000. The AlS was coded reviewers(20% of the Each Movie data set, randomly chosen) to provide predictions and recommendations for 100 users Averaged statistics are then taken for each run. Runtimes ranged from 5 to 60 minutes, largely dependent on the Effect of stimulation on recommendation accuracy number of reviewers Experiments on Simple AlS Initial experiments concentrated on a simple AlS, with diotypic effects. The goal was to find a good stimulation ii rate, but also to ensure that the baseline'system operates i similarly to a Simple Pearson predictor (SP). Therefore, we set the suppression rate to zero, and varied only the stimulation rate, i.e. the weighting given to antigen binding 0.2406 Other parameters had been fixed by preliminary experiments EfLect of stimulation on number of recommendations Effect of Stimultion on Neighbourhood size Sumulation

counting the number of discordant pairs. To do this we order the films by vote and apply the following formulae: ( ) ( ) ( )    > = = − = − ∑ ∑= + = otherwise if r r D r r N D r r n n N i j i j n i n j i D i j D 0 1 , , )6( 1 4 1 1 1 τ Where n is the overlap size and ri is the rank of film i as recommended by the neighbourhood. Note that i here refers to the antigen rank of the film, not the film ID. ND is the number of discordant pairs, or, equivalently, the expected cost of a bubble sort to reconcile the two lists. D is set to one if the rankings are discordant. Mean number of reviewers. This is the number of reviewers looked at before the AIS stabilised. Mean number of neighbours: This is the final number of neighbours in the stabilised AIS. 3 EXPERIMENTS Experiments were carried out on a Pentium 700 with 256MB RAM, running Windows 2000. The AIS was coded in JavaTM JDK1.3. Each run involved looking at up to 15,000 reviewers (20% of the EachMovie data set, randomly chosen) to provide predictions and recommendations for 100 users. Averaged statistics are then taken for each run. Runtimes ranged from 5 to 60 minutes, largely dependent on the number of reviewers. Experiments on Simple AIS Initial experiments concentrated on a simple AIS, with no idiotypic effects. The goal was to find a good stimulation rate, but also to ensure that the ‘baseline’ system operates similarly to a Simple Pearson predictor (SP). Therefore, we set the suppression rate to zero, and varied only the stimulation rate, i.e. the weighting given to antigen binding. Other parameters had been fixed by preliminary experiments. Effect of Stimulation on Neighbourhood size 0 10 20 30 40 50 60 70 80 90 100 0 0.2 0.4 0.6 0.8 1 Stimulation Rate Neighbourhood Size Effect of stimulation on number of users looked at 0 5000 10000 15000 0 0.2 0.4 0.6 0.8 1 Stimulation Rate Num ber of users looked at Figure 1: Effect of stimulation rate on neighbourhood and reviewers. The graphs show averaged results over five runs at each stimulation rate. The bars show standard deviations. In order to have a fair comparison, the Simple Pearson parameters (neighbourhood and number of reviewers looked at) match the AIS values for each rate. In figure 2, we show the prediction error, number of recommendations, number of overlaps and recommendation accuracy for each algorithm. Note that low prediction error values are better, whereas for the other measures we are looking for high values. Effect of stimulation on prediction error 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 0 0.2 0.4 0.6 0.8 1 Stimulation Rate M ean Absolute E rror AIS (av) SP (av) Effect of stimulation on recommendation accuracy 0.35 0.4 0.45 0.5 0.55 0 0.2 0.4 0.6 0.8 1 Stimulation Rate Recommendation Accuracy (Kendall's Tau) AIS (av) SP (av) Effect of Stimulation on number of recommendations 0 200 400 600 800 1000 1200 0 0.2 0.4 0.6 0.8 1 Stimulation Rate Number of recommendations AIS(av) SP (av)

Effect of stimulation on number of overlaps Effect of suppression on number of reviewers looked at Figure 2: Effect of stimulation rate on prediction and recommendation Figure 3: Effect of suppression rate on neighbourhood size and reviewers It can be seen that the simple AIS gives broadly similar We then tested the effect of suppression on the Als prediction performance to the Simple Pearson. The MAE performance. Here we fixed the baseline rate at stimulation measurements from different runs are not normally only(no suppression), and took measurements relative to this distributed,so a non-parametric statistic is appropriate. We baseline. Again, it should be noted that the first graph shows performed a Wilcoxon analysis, which showed that the prediction error(hence, a good result is low) fference between prediction errors of SP and Als is zero with 95% confidence. In addition, the choice of an appropriate stimulation rate did make a significant difference (a rate of 0.2 compared with 0.02 at the 95% level) For recommendation, the AlS performs better than the SP positive95% Wilcoxon analysis to assess significance. We itmomft t at stimulation rates above 0. 1. Again, we performed a excluded cases where a recommendation score was unavailable(due to an insufficient number of overlaps ) The sow number of recommendations and overlaps show similar trends though the als gives a more constant value. Again, some stimulation was beneficial In later experiments, the stimulation rate was fixed at one of the better values(0. 2, 0.3 or 0.5), in order to give us a Effect of suppression on recommendation accuracy good base to work on. These values give us generally good performance, while keeping a good neighbourhood size and still evaluating a reasonable number of reviewers 110.0% Experiments on the Idiotypic AlS Having fixed all the simple parameters, we tested of suppression for stimulation rates of 0. 2, 0.3 and surprisingly we found that suppression changed the d os. not of reviewers looked at and the num ber of neighbours Effect of suppression on neighbourhood siz Effect of suppression on number of overlaps

Effect of Stimulation on number of overlaps 0 10 20 30 40 50 60 0 0.2 0.4 0.6 0.8 1 Stimulation Number of overlaps AIS(av) SP (av) Figure 2: Effect of stimulation rate on prediction and recommendation. It can be seen that the simple AIS gives broadly similar prediction performance to the Simple Pearson. The MAE measurements from different runs are not normally distributed, so a non-parametric statistic is appropriate. We performed a Wilcoxon analysis, which showed that the difference between prediction errors of SP and AIS is zero with 95% confidence. In addition, the choice of an appropriate stimulation rate did make a significant difference (a rate of 0.2 compared with 0.02 at the 95% level). For recommendation, the AIS performs better than the SP at stimulation rates above 0.1. Again, we performed a positive 95% Wilcoxon analysis to assess significance. We excluded cases where a recommendation score was unavailable (due to an insufficient number of overlaps). The number of recommendations and overlaps show similar trends though the AIS gives a more constant value. Again, some stimulation was beneficial. In later experiments, the stimulation rate was fixed at one of the better values (0.2, 0.3 or 0.5), in order to give us a good base to work on. These values give us generally good performance, while keeping a good neighbourhood size and still evaluating a reasonable number of reviewers. Experiments on the Idiotypic AIS Having fixed all the simple parameters, we tested the effect of suppression for stimulation rates of 0.2, 0.3 and 0.5. Not surprisingly we found that suppression changed the number of reviewers looked at and the number of neighbours: Effect of suppression on neighbourhood size 0 10 20 30 40 50 60 70 80 90 100 0 0.2 0.4 0.6 0.8 1 Suppression rate Neighbourhood size Rate 0.2 Rate 0.3 Rate 0.5 Effect of suppression on number of reviewers looked at 0 2000 4000 6000 8000 10000 12000 14000 16000 0 0.2 0.4 0.6 0.8 1 Suppression Rate Number reviewers Rate 0.2 Rate 0.3 Rate 0.5 Figure 3: Effect of suppression rate on neighbourhood size and reviewers. We then tested the effect of suppression on the AIS performance. Here we fixed the baseline rate at stimulation only (no suppression), and took measurements relative to this baseline. Again, it should be noted that the first graph shows prediction error (hence, a good result is low). Effect of suppression on prediction error 70.0% 80.0% 90.0% 100.0% 110.0% 120.0% 130.0% 0 0.2 0.4 0.6 0.8 1 Suppression rate Mean absolute error (relative to baseline ) Rate 0.2 Rate 0.3 Rate 0.5 Effect of suppression on recommendation accuracy 80.0% 90.0% 100.0% 110.0% 120.0% 0 0.2 0.4 0.6 0.8 1 Suppression rate Recommendation accuracy (Kendall) relative to baseline Rate 0.2 Rate 0.3 Rate 0.5 Effect of suppression on number of overlaps 0.0% 20.0% 40.0% 60.0% 80.0% 100.0% 0 0.2 0.4 0.6 0.8 1 Suppression rate Number of overlaps (relative to baseline) Rate 0.2 Rate 0.3 Rate 0.5

Effect of suppression on number of recommend a small amount of suppression may indeed be beneficial to AIs performance, in particular recommendation. It is interesting to note that the increase in recommendation quality occurs with a relatively constant overlap size. At too high levels of suppression, it is harder to fill the neighbourhood, with consequent lack of diversity and hence ecommendation accuracy We believe that these initial results show two things Firstly, population effects can be beneficial for CI 002040608 algorithms, particularly for recommendation; secondly, that CF is a promising new application area for artificial immune Figure 4: Effect of suppression rate on prediction and recommendation systems. In fact, we can widen the context, since the process of neighbourhood selection described in this paper can easily Again, the graphs show averaged results over five runs at be generalized to the task of ad-hoc community formation. each suppression rate. 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[5] Compaq Systems Research Centre. Ea collaborative filtering lataset(http://www.research.compaq.com/src/eachmOvi [6 Delgado J, Ishii N and Tomoki U. Content-based Collaborative 4. CONCLUSIONS Information Filtering: Actively Learning to Classify and Recommend Documents. In: Cooperative Information Agents Il Learning, Mobility erce for Information Discovery on the Internet, It is not particularly surprising that the simple AlS performs similarly to the SP predictor. This is because they are, at their [7 Delgado J. and Ishii, Multi-agent Learning in Recommender System ore based around the same algorithm. The stimulation rate Information Systems, vol. 10, Pp. 81-100, 2001 (in absence of any idiotypic effect)is effectively setting a [ 8] Farmer JD, Packard NH and Perelson AS,The threshold for correlation. This has both strengths and adaptation, and machine learning Physica, vol. 22, pp. 187-204, 1986 weaknesses. It has been shown that a threshold is useful in 1 Fisher D, Hildrum K, Hong J, Newman M and Vuduc R, SWAMI: a discarding the potentially misleading predictions of poorly framework for collaborative filtering algorithm development and evaluation1999.http://guir.berkeley.edu/projects/swami/ correlated reviewers [10]. On the other hand, a rigid [10] Gokhale A, Improvements to Collaborative Filtering Algorithms 1999 threshold means that one has to 'prejudge' the appropriate Worcester Polytech Ite.http://www.cs.wpi.edu/-claypool/ms level to avoid both premature convergence communities. Indeed, detailed examination of the individual [11] Hightower RR, Forrest S and Perelson AS "The evolution of emergent neighbourhood either early or not at all. The setting of a四m,cmCe)P3/的 rganization in immune system gene libraries, " Proceedings of the 6th mational Confere threshold also means that sufficiently good antibodies are 1996 taken on a first come, first served basis. It is interesting to [13 Jene nK, Towards a network theory of the immune system Annals of observe that such a strategy nevertheless seems (in these Immunology, vol. 125, no C, pp 373-389, 1973 experiments) to provide a more constant level of overlaps, [14] Kirkwood E and Lewis C Understanding Medical Immunology, John and better recommendation quality wiley Sons, Chichester, 1989 The richness of our AIS model comes when we allow [15] Perelson AS and Weisbuch G, Immunology for physicists Reviews of Modern Physics, voL. 69, pp. 1219-1267, 1997 nteractions betwe antibodies. Early ualitative [16 Resnick P and Varian HR, Recommender systems Communications experimentation with the idiotypic network showed antibody the ACM, vol. 40, pp. 56-58, 1997. concentration rising and falling dynamically as the population varied. For instance, in the simple AlS, the concentration of an antibody will monotonically increase to saturation, or decrease to elimination, unaffected by the other antibodies. however there is a delicate balance to be struck between stimulation and suppression. An imbalance may lead to a loss in population size or diversity. The graphs show that

Effect of suppression on number of recommendations 0.0% 20.0% 40.0% 60.0% 80.0% 100.0% 120.0% 0 0.2 0.4 0.6 0.8 1 Suppression rate Number of recommendations (relative to baseline) Rate 0.2 Rate 0.3 Rate 0.5 Figure 4: Effect of suppression rate on prediction and recommendation. Again, the graphs show averaged results over five runs at each suppression rate. The bars show standard deviations (similar size bars for rates 0.2 and 0.5 have been omitted in the interests of clarity). At low levels of stimulation, prediction accuracy is not significantly affected. However recommendation accuracy is improved significantly (95% Wilcoxon). For instance, for 0.3 stimulation, rates from 0.05 to 0.2 gave a significantly improved performance. In actual terms, the Kendall measure rises from 0.5 to nearly 0.6. This means that the chance of any two randomly sampled pairs being correctly ranked has risen from 60% to 80%. Too much suppression had a detrimental effect on all measures. 4. CONCLUSIONS It is not particularly surprising that the simple AIS performs similarly to the SP predictor. This is because they are, at their core, based around the same algorithm. The stimulation rate (in absence of any idiotypic effect) is effectively setting a threshold for correlation. This has both strengths and weaknesses. It has been shown that a threshold is useful in discarding the potentially misleading predictions of poorly correlated reviewers [10]. On the other hand, a rigid threshold means that one has to ‘prejudge’ the appropriate level to avoid both premature convergence and empty communities. Indeed, detailed examination of the individual runs showed that the AIS had a tendency to fill its neighbourhood either early or not at all. The setting of a threshold also means that sufficiently good antibodies are taken on a first come, first served basis. It is interesting to observe that such a strategy nevertheless seems (in these experiments) to provide a more constant level of overlaps, and better recommendation quality. The richness of our AIS model comes when we allow interactions between antibodies. Early, qualitative experimentation with the idiotypic network showed antibody concentration rising and falling dynamically as the population varied. For instance, in the simple AIS, the concentration of an antibody will monotonically increase to saturation, or decrease to elimination, unaffected by the other antibodies. However, there is a delicate balance to be struck between stimulation and suppression. An imbalance may lead to a loss in population size or diversity. The graphs show that a small amount of suppression may indeed be beneficial to AIS performance, in particular recommendation. It is interesting to note that the increase in recommendation quality occurs with a relatively constant overlap size. At too high levels of suppression, it is harder to fill the neighbourhood, with consequent lack of diversity and hence recommendation accuracy. We believe that these initial results show two things. Firstly, population effects can be beneficial for CF algorithms, particularly for recommendation; secondly, that CF is a promising new application area for artificial immune systems. In fact, we can widen the context, since the process of neighbourhood selection described in this paper can easily be generalized to the task of ad-hoc community formation. REFERENCES [1] Aggarwal C and Yu P, On Text Mining Techniques for Personalization Lecture Notes in Artificial Intelligence, vol. 1711, pp. 12-18, 1999. [2] Amazon.com Recommendations (http://www.amazon.com/ /). [3] Billsus, D. and Pazzani, M. J., "Learning Collaborative Information Filters," Proceedings of the Fifteenth International Conference on Machine Learning. pp. 46-54, 1998. [4] Breese JS, Heckerman D and Kadie C, Empirical Analysis of predictive algorithms for collaborative filtering, Proceedings of the 14 Conference on Uncertainty in Reasoning, pp. 43-52, 1998. [5] Compaq Systems Research Centre. EachMovie collaborative filtering data set (http://www.research.compaq.com/SRC/eachmovie/). [6] Delgado J, Ishii N and Tomoki U. Content-based Collaborative Information Filtering: Actively Learning to Classify and Recommend Documents. In: Cooperative Information Agents II. Learning, Mobility and Electronic Commerce for Information Discovery on the Internet, ed. M. Klusch, G. W. E. Springer-Verlag, 1998. [7] Delgado J. and Ishii, Multi-agent Learning in Recommender Systems For Information Filtering on the Internet Journal of Co-operative Information Systems, vol. 10, pp. 81-100, 2001. [8] Farmer JD, Packard NH and Perelson AS, The immune system, adaptation, and machine learning Physica, vol. 22, pp. 187-204, 1986. [9] Fisher D, Hildrum K, Hong J, Newman M and Vuduc R, SWAMI: a framework for collaborative filtering algorithm development and evaluation 1999. http://guir.berkeley.edu/projects/swami/. [10] Gokhale A, Improvements to Collaborative Filtering Algorithms 1999. Worcester Polytechnic Institute. http://www.cs.wpi.edu/~claypool/ms /cf-improve/. [11] Hightower RR, Forrest S and Perelson AS. "The evolution of emergent organization in immune system gene libraries," Proceedings of the 6th International Conference on Genetic Algorithms, pp. 344--350, 1995. [12] Hunt J, King C and Cooke D, Immunizing against fraud, IEEE Colloquium on Knowledge Discovery and Data Mining, vol. 4, pp. 1-4, 1996. [13] Jerne NK, Towards a network theory of the immune system Annals of Immunology, vol. 125, no. C, pp. 373-389, 1973. [14] Kirkwood E and Lewis C. Understanding Medical Immunology, John Wiley & Sons, Chichester, 1989. [15] Perelson AS and Weisbuch G, Immunology for physicists Reviews of Modern Physics, vol. 69, pp. 1219-1267, 1997. [16] Resnick P and Varian HR, Recommender systems Communications of the ACM, vol. 40, pp. 56-58, 1997