The quality of such an asymptotically stable partition provides an upper bound on the quality of an optimal partition. Hence, the Leiden algorithm effectively addresses the problem of badly connected communities. The solution proposed in smart local moving is to alter how the local moving step in Louvain works. Speed of the first iteration of the Louvain and the Leiden algorithm for benchmark networks with increasingly difficult partitions (n=107). Unsupervised clustering of cells is a common step in many single-cell expression workflows. The Louvain algorithm guarantees that modularity cannot be increased by merging communities (it finds a locally optimal solution). It maximizes a modularity score for each community, where the modularity quantifies the quality of an assignment of nodes to communities. Node mergers that cause the quality function to decrease are not considered. IEEE Trans. To address this problem, we introduce the Leiden algorithm. Rev. E 92, 032801, https://doi.org/10.1103/PhysRevE.92.032801 (2015). Google Scholar. ACM Trans. 2015. This problem is different from the well-known issue of the resolution limit of modularity14. Leiden keeps finding better partitions for empirical networks also after the first 10 iterations of the algorithm. Lancichinetti, A. Guimer, R. & Nunes Amaral, L. A. Functional cartography of complex metabolic networks. For lower values of , the correct partition is easy to find and Leiden is only about twice as fast as Louvain. Edges were created in such a way that an edge fell between two communities with a probability and within a community with a probability 1. However, as increases, the Leiden algorithm starts to outperform the Louvain algorithm. The constant Potts model (CPM), so called due to the use of a constant value in the Potts model, is an alternative objective function for community detection. Note that communities found by the Leiden algorithm are guaranteed to be connected. Good, B. H., De Montjoye, Y. Although originally defined for modularity, the Louvain algorithm can also be used to optimise other quality functions. In this new situation, nodes 2, 3, 5 and 6 have only internal connections. Proc. Google Scholar. reviewed the manuscript. PubMed Modularity is a measure of the structure of networks or graphs which measures the strength of division of a network into modules (also called groups, clusters or communities). Both conda and PyPI have leiden clustering in Python which operates via iGraph. In this post Ive mainly focused on the optimisation methods for community detection, rather than the different objective functions that can be used. J. This is the crux of the Leiden paper, and the authors show that this exact problem happens frequently in practice. Louvain community detection algorithm was originally proposed in 2008 as a fast community unfolding method for large networks. Here we can see partitions in the plotted results. Leiden is both faster than Louvain and finds better partitions. The Louvain algorithm is illustrated in Fig. Again, if communities are badly connected, this may lead to incorrect inferences of topics, which will affect bibliometric analyses relying on the inferred topics. For the results reported below, the average degree was set to \(\langle k\rangle =10\). The R implementation of Leiden can be run directly on the snn igraph object in Seurat. wrote the manuscript. MathSciNet Conversely, if Leiden does not find subcommunities, there is no guarantee that modularity cannot be increased by splitting up the community. Inf. Based on this partition, an aggregate network is created (c). Faster Unfolding of Communities: Speeding up the Louvain Algorithm. Phys. Rev. Thank you for visiting nature.com. leiden_clustering Description Class wrapper based on scanpy to use the Leiden algorithm to directly cluster your data matrix with a scikit-learn flavor. We find that the Leiden algorithm commonly finds partitions of higher quality in less time. Based on project statistics from the GitHub repository for the PyPI package leiden-clustering, we found that it has been starred 1 times. Traag, V A. SPATA2 currently offers the functions findSeuratClusters (), findMonocleClusters () and findNearestNeighbourClusters () which are wrapper around widely used clustering algorithms. As such, we scored leiden-clustering popularity level to be Limited. We here introduce the Leiden algorithm, which guarantees that communities are well connected. Importantly, the output of the local moving stage will depend on the order that the nodes are considered in. The degree of randomness in the selection of a community is determined by a parameter >0. One of the most widely used algorithms is the Louvain algorithm10, which is reported to be among the fastest and best performing community detection algorithms11,12. This continues until the queue is empty. Such a modular structure is usually not known beforehand. Electr. and L.W. where >0 is a resolution parameter4. Furthermore, by relying on a fast local move approach, the Leiden algorithm runs faster than the Louvain algorithm. These steps are repeated until the quality cannot be increased further. The percentage of disconnected communities is more limited, usually around 1%. Four popular community detection algorithms are explained . Obviously, this is a worst case example, showing that disconnected communities may be identified by the Louvain algorithm. Phys. For the Amazon and IMDB networks, the first iteration of the Leiden algorithm is only about 1.6 times faster than the first iteration of the Louvain algorithm. Therefore, clustering algorithms look for similarities or dissimilarities among data points. In fact, when we keep iterating the Leiden algorithm, it will converge to a partition for which it is guaranteed that: A community is uniformly -dense if there are no subsets of the community that can be separated from the community. Cluster cells using Louvain/Leiden community detection Description. The parameter functions as a sort of threshold: communities should have a density of at least , while the density between communities should be lower than . In that case, some optimal partitions cannot be found, as we show in SectionC2 of the Supplementary Information. This phenomenon can be explained by the documented tendency KMeans has to identify equal-sized , combined with the significant class imbalance associated with the datasets having more than 8 clusters (Table 1). Klavans, R. & Boyack, K. W. Which Type of Citation Analysis Generates the Most Accurate Taxonomy of Scientific and Technical Knowledge? The horizontal axis indicates the cumulative time taken to obtain the quality indicated on the vertical axis. Besides the relative flexibility of the implementation, it also scales well, and can be run on graphs of millions of nodes (as long as they can fit in memory). This may have serious consequences for analyses based on the resulting partitions. Ozaki, Naoto, Hiroshi Tezuka, and Mary Inaba. Modularity is a popular objective function used with the Louvain method for community detection. A smart local moving algorithm for large-scale modularity-based community detection. Trying to fix the problem by simply considering the connected components of communities19,20,21 is unsatisfactory because it addresses only the most extreme case and does not resolve the more fundamental problem. Consider the partition shown in (a). The images or other third party material in this article are included in the articles Creative Commons license, unless indicated otherwise in a credit line to the material. A number of iterations of the Leiden algorithm can be performed before the Louvain algorithm has finished its first iteration. Randomness in the selection of a community allows the partition space to be explored more broadly. Indeed, the percentage of disconnected communities becomes more comparable to the percentage of badly connected communities in later iterations. The algorithm may yield arbitrarily badly connected communities, over and above the well-known issue of the resolution limit14. However, focussing only on disconnected communities masks the more fundamental issue: Louvain finds arbitrarily badly connected communities. Presumably, many of the badly connected communities in the first iteration of Louvain become disconnected in the second iteration. Correspondence to Importantly, the problem of disconnected communities is not just a theoretical curiosity. They show that the original Louvain algorithm that can result in badly connected communities (even communities that are completely disconnected internally) and propose an alternative method, Leiden, that guarantees that communities are well connected. In the Louvain algorithm, a node may be moved to a different community while it may have acted as a bridge between different components of its old community. Requirements Developed using: scanpy v1.7.2 sklearn v0.23.2 umap v0.4.6 numpy v1.19.2 leidenalg Installation pip pip install leiden_clustering local Badly connected communities. 92 (3): 032801. http://dx.doi.org/10.1103/PhysRevE.92.032801. We study the problem of badly connected communities when using the Louvain algorithm for several empirical networks. The authors act as bibliometric consultants to CWTS B.V., which makes use of community detection algorithms in commercial products and services. This algorithm provides a number of explicit guarantees. Uniform -density means that no matter how a community is partitioned into two parts, the two parts will always be well connected to each other. J. Stat. import leidenalg as la import igraph as ig Example output. Brandes, U. et al. Removing such a node from its old community disconnects the old community. The 'devtools' package will be used to install 'leiden' and the dependancies (igraph and reticulate). This is because Louvain only moves individual nodes at a time. For example, nodes in a community in biological or neurological networks are often assumed to share similar functions or behaviour25. Rev. Elect. Soft Matter Phys. Then optimize the modularity function to determine clusters. Node optimality is also guaranteed after a stable iteration of the Louvain algorithm. Not. To address this important shortcoming, we introduce a new algorithm that is faster, finds better partitions and provides explicit guarantees and bounds. Int. Louvain algorithm. performed the experimental analysis. In particular, we show that Louvain may identify communities that are internally disconnected. A community size of 50 nodes was used for the results presented below, but larger community sizes yielded qualitatively similar results. B 86 (11): 471. https://doi.org/10.1140/epjb/e2013-40829-0. 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We generated networks with n=103 to n=107 nodes. We gratefully acknowledge computational facilities provided by the LIACS Data Science Lab Computing Facilities through Frank Takes. Neurosci. The percentage of disconnected communities even jumps to 16% for the DBLP network. This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository. The authors show that the total computational time for Louvain depends a lot on the number of phase one loops (loops during the first local moving stage). Two ways of doing this are graph modularity (Newman and Girvan 2004) and the constant Potts model (Ronhovde and Nussinov 2010). Higher resolutions lead to more communities, while lower resolutions lead to fewer communities. First iteration runtime for empirical networks. Community detection in complex networks using extremal optimization. Blondel, V. D., Guillaume, J.-L., Lambiotte, R. & Lefebvre, E. Fast unfolding of communities in large networks. The problem of disconnected communities has been observed before19,20, also in the context of the label propagation algorithm21. Subpartition -density is not guaranteed by the Louvain algorithm. However, the Louvain algorithm does not consider this possibility, since it considers only individual node movements. CPM does not suffer from this issue13. Rev. & Clauset, A. We find that the Leiden algorithm is faster than the Louvain algorithm and uncovers better partitions, in addition to providing explicit guarantees. b, The elephant graph (in a) is clustered using the Leiden clustering algorithm 51 (resolution r = 0.5). For each community, modularity measures the number of edges within the community and the number of edges going outside the community, and gives a value between -1 and +1. Runtime versus quality for empirical networks. Phys. In contrast, Leiden keeps finding better partitions in each iteration. 2016. In short, the problem of badly connected communities has important practical consequences. When a disconnected community has become a node in an aggregate network, there are no more possibilities to split up the community. Even worse, the Amazon network has 5% disconnected communities, but 25% badly connected communities. The algorithm then moves individual nodes in the aggregate network (e). However, it is also possible to start the algorithm from a different partition15. The increase in the percentage of disconnected communities is relatively limited for the Live Journal and Web of Science networks. Here is some small debugging code I wrote to find this. Louvain has two phases: local moving and aggregation. After a stable iteration of the Leiden algorithm, the algorithm may still be able to make further improvements in later iterations. Algorithmics 16, 2.1, https://doi.org/10.1145/1963190.1970376 (2011). These nodes are therefore optimally assigned to their current community. I tracked the number of clusters post-clustering at each step. This should be the first preference when choosing an algorithm. Speed of the first iteration of the Louvain and the Leiden algorithm for six empirical networks. All authors conceived the algorithm and contributed to the source code. 5, for lower values of the partition is well defined, and neither the Louvain nor the Leiden algorithm has a problem in determining the correct partition in only two iterations. Article Analyses based on benchmark networks have only a limited value because these networks are not representative of empirical real-world networks. . For each set of parameters, we repeated the experiment 10 times. One of the most popular algorithms to optimise modularity is the so-called Louvain algorithm10, named after the location of its authors. Sci. At some point, the Louvain algorithm may end up in the community structure shown in Fig. Rev. However, if communities are badly connected, this may lead to incorrect attributions of shared functionality. Nodes 16 have connections only within this community, whereas node 0 also has many external connections. 1 and summarised in pseudo-code in AlgorithmA.1 in SectionA of the Supplementary Information. There was a problem preparing your codespace, please try again. In our experimental analysis, we observe that up to 25% of the communities are badly connected and up to 16% are disconnected. Rev. This is very similar to what the smart local moving algorithm does. The phase one loop can be greatly accelerated by finding the nodes that have the potential to change community and only revisit those nodes. Data Eng. This makes sense, because after phase one the total size of the graph should be significantly reduced. In the case of modularity, communities may have significant substructure both because of the resolution limit and because of the shortcomings of Louvain. As can be seen in the figure, Louvain quickly reaches a state in which it is unable to find better partitions. CAS Article Phys. Article The two phases are repeated until the quality function cannot be increased further. The Leiden community detection algorithm outperforms other clustering methods. A. In addition, we prove that the algorithm converges to an asymptotically stable partition in which all subsets of all communities are locally optimally assigned. In this case, refinement does not change the partition (f). While smart local moving and multilevel refinement can improve the communities found, the next two improvements on Louvain that Ill discuss focus on the speed/efficiency of the algorithm. The Leiden algorithm is partly based on the previously introduced smart local move algorithm15, which itself can be seen as an improvement of the Louvain algorithm. In addition, to analyse whether a community is badly connected, we ran the Leiden algorithm on the subnetwork consisting of all nodes belonging to the community. The Louvain local moving phase consists of the following steps: This process is repeated for every node in the network until no further improvement in modularity is possible. Leiden is faster than Louvain especially for larger networks. 10, 186198, https://doi.org/10.1038/nrn2575 (2009). This represents the following graph structure. The percentage of badly connected communities is less affected by the number of iterations of the Louvain algorithm. As discussed earlier, the Louvain algorithm does not guarantee connectivity. The idea of the refinement phase in the Leiden algorithm is to identify a partition \({{\mathscr{P}}}_{{\rm{refined}}}\) that is a refinement of \({\mathscr{P}}\). Rev. E 81, 046106, https://doi.org/10.1103/PhysRevE.81.046106 (2010). It is a directed graph if the adjacency matrix is not symmetric. Similarly, in citation networks, such as the Web of Science network, nodes in a community are usually considered to share a common topic26,27. Each of these can be used as an objective function for graph-based community detection methods, with our goal being to maximize this value. We abbreviate the leidenalg package as la and the igraph package as ig in all Python code throughout this documentation. Article https://doi.org/10.1038/s41598-019-41695-z, DOI: https://doi.org/10.1038/s41598-019-41695-z. Phys. CAS The resulting clusters are shown as colors on the 3D model (top) and t -SNE embedding . We now compare how the Leiden and the Louvain algorithm perform for the six empirical networks listed in Table2. In this case we know the answer is exactly 10. Hence, the community remains disconnected, unless it is merged with another community that happens to act as a bridge. 8 (3): 207. https://pdfs.semanticscholar.org/4ea9/74f0fadb57a0b1ec35cbc5b3eb28e9b966d8.pdf. 2008. As can be seen in Fig. The refined partition \({{\mathscr{P}}}_{{\rm{refined}}}\) is obtained as follows. Traag, V. A., Waltman, L. & van Eck, N. J. networkanalysis. Phys. Powered by DataCamp DataCamp The algorithm moves individual nodes from one community to another to find a partition (b). This way of defining the expected number of edges is based on the so-called configuration model. One of the best-known methods for community detection is called modularity3. Lancichinetti, A., Fortunato, S. & Radicchi, F. Benchmark graphs for testing community detection algorithms. Article The numerical details of the example can be found in SectionB of the Supplementary Information. It was originally developed for modularity optimization, although the same method can be applied to optimize CPM. This enables us to find cases where its beneficial to split a community.