1. School of Information Science and Technology, University of Science and Technology of China, Hefei 230026, China 2. School of Computer Science and Technology, Zhejiang University, Hangzhou 310058, China 3. School of Computer Science and Technology, Shandong University, Qingdao 250100, China 4. Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
Graph convolutional networks (GCNs) have become prevalent in recommender system (RS) due to their superiority in modeling collaborative patterns. Although improving the overall accuracy, GCNs unfortunately amplify popularity bias — tail items are less likely to be recommended. This effect prevents the GCN-based RS from making precise and fair recommendations, decreasing the effectiveness of recommender systems in the long run.
In this paper, we investigate how graph convolutions amplify the popularity bias in RS. Through theoretical analyses, we identify two fundamental factors: (1) with graph convolution (i.e., neighborhood aggregation), popular items exert larger influence than tail items on neighbor users, making the users move towards popular items in the representation space; (2) after multiple times of graph convolution, popular items would affect more high-order neighbors and become more influential. The two points make popular items get closer to almost users and thus being recommended more frequently. To rectify this, we propose to estimate the amplified effect of popular nodes on each node’s representation, and intervene the effect after each graph convolution. Specifically, we adopt clustering to discover highly-influential nodes and estimate the amplification effect of each node, then remove the effect from the node embeddings at each graph convolution layer. Our method is simple and generic — it can be used in the inference stage to correct existing models rather than training a new model from scratch, and can be applied to various GCN models. We demonstrate our method on two representative GCN backbones LightGCN and UltraGCN, verifying its ability in improving the recommendations of tail items without sacrificing the performance of popular items. Codes are open-sourced
The embedding of user or item at the th graph convolution layer
The individual loss term of an interaction
A set of nodes in the -th cluster obtained by using Kmeans given the embeddings
A set of nodes
A set of nodes
The pooling representations after normalization of
The pooling representations after normalization of
Tab.1
Fig.2
Fig.3
Dataset
#Users
#Items
#Interactions
Density
Gowalla
29,858
40,981
1,027,370
0.00084
Yelp2018
31,668
38,048
1,561,406
0.00130
Amazon-book
52,643
91,599
2,984,108
0.00062
Tab.2
Dataset
Gowalla
Yelp2018
Amazon-book
Models
Overall
Tail
Overall
Tail
Overall
Tail
Recall
NDCG
Recall
NDCG
Recall
NDCG
Recall
NDCG
Recall
NDCG
Recall
NDCG
LightGCN
0.1820
0.1546
0.0434
0.0191
0.0627
0.0516
0.0091
0.0046
0.0414
0.0321
0.009
0.0051
BFGCN
0.1083
0.0805
0.0468
0.0245
0.0389
0.0311
0.0124
0.0076
0.0276
0.0211
0.0097
0.0059
LightGCN-IPSCN
0.1325
0.1132
0.0477
0.0213
0.0473
0.0391
0.0136
0.0077
0.0285
0.0221
0.0118
0.0069
LightGCN-CausE
0.1334
0.1137
0.0485
0.0225
0.0492
0.0405
0.0141
0.0085
0.0299
0.0230
0.0127
0.0078
LightGCN-DICE
0.1337
0.1138
0.0493
0.0241
0.0505
0.0409
0.0132
0.0073
0.0348
0.0264
0.0121
0.0074
LightGCN-MACR
0.1188
0.0928
0.0478
0.0219
0.0343
0.027
0.0233
0.0126
0.0269
0.0204
0.0108
0.0065
LightGCN-Tail
0.1647
0.1391
0.0628
0.0319
0.057
0.0466
0.0154
0.0095
0.0369
0.0283
0.0151
0.0094
LightGCN-BxQuAD
0.1378
0.1130
0.0689
0.0360
0.0545
0.0431
0.0209
0.0123
0.0389
0.0304
0.0164
0.0108
LightGCN-DAP-o
0.1834
0.1564
0.0538
0.0245
0.0634
0.0521
0.0137
0.0073
0.0436
0.0339
0.0134
0.0079
LightGCN-DAP-t
0.1672
0.1427
0.0708
0.0354
0.0562
0.0461
0.0218
0.0129
0.0414
0.0328
0.0166
0.0102
Improve/%
0.77
1.16
23.96
28.27
1.12
0.97
50.55
58.70
4.83
5.61
48.89
54.90
Tab.3
Dataset
Gowalla
Yelp2018
Amazon-book
Models
Overall
Tail
Overall
Tail
Overall
Tail
Recall
NDCG
Recall
NDCG
Recall
NDCG
Recall
NDCG
Recall
NDCG
Recall
NDCG
UltraGCN
0.1862
0.1579
0.0447
0.0213
0.0676
0.0554
0.0127
0.0074
0.0682
0.0556
0.0436
0.0297
UltraGCN-IPSCN
0.1345
0.1123
0.0451
0.0208
0.0401
0.0324
0.0144
0.0087
0.0442
0.0356
0.0458
0.0317
UltraGCN-CausE
0.1408
0.1177
0.0449
0.0209
0.0411
0.0329
0.0151
0.0096
0.0459
0.0369
0.0463
0.0320
UltraGCN-DICE
0.1424
0.1201
0.0512
0.0247
0.0516
0.0417
0.0157
0.0096
0.0545
0.0423
0.0491
0.0343
UltraGCN-MACR
0.1311
0.1078
0.0517
0.0252
0.0387
0.0323
0.0248
0.0141
0.0501
0.0398
0.0488
0.0335
UltraGCN-Tail
0.1788
0.1521
0.0634
0.0321
0.0618
0.0501
0.0167
0.0102
0.0599
0.0499
0.0531
0.0378
UltraGCN-BxQuAD
0.1482
0.1289
0.0694
0.0361
0.0591
0.0482
0.0218
0.0136
0.0623
0.0517
0.0547
0.0386
UltraGCN-DAP-o
0.1868
0.1580
0.0551
0.0271
0.0678
0.0555
0.0135
0.0079
0.0688
0.0562
0.0462
0.0316
UltraGCN-DAP-t
0.1701
0.1483
0.0714
0.0362
0.0607
0.0493
0.0237
0.0135
0.0625
0.0520
0.0543
0.0391
Improve/%
0.32
0.06
5.59
6.57
0.30
0.18
6.30
6.76
0.88
1.07
5.96
6.40
Tab.4
Fig.4
Fig.5
Dataset
Gowalla
Yelp2018
Amazon-book
P
Recall
NDCG
Recall
NDCG
Recall
NDCG
1
0.1819
0.1547
0.0629
0.0517
0.0423
0.0329
5
0.182
0.1548
0.063
0.0518
0.0436
0.0339
10
0.1823
0.1552
0.0633
0.0521
0.0435
0.0339
30
0.1831
0.1563
0.0628
0.0518
0.0435
0.0338
50
0.1834
0.1564
0.0626
0.0517
0.0428
0.0334
70
0.1833
0.1564
0.0623
0.0515
0.0426
0.0331
Tab.5
Fig.6
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