Detecting the spatiotemporal characteristics of the supply-chain disruption and estimating its short term effects Giannis Spiliopoulos∗ César Ducruet∗ Leonardo M. Millefiori∗ Intelligent Transportation Systems French National Center for Scientific NATO STO Centre for Maritime Lab, University of the Aegean Research (CNRS), UMR 7235 Research and Experimentation Ermoupolis, Syros, Greece EconomiX, University of Paris (CMRE) gspiliopoulos@aegean.gr Nanterre, France La Spezia, Italy cesar.ducruet@economix.fr leonardo.millefiori@cmre.nato.int Paolo Braca∗ Dimitris Zissis†∗ NATO STO Centre for Maritime Intelligent Transportation Systems Research and Experimentation Lab, University of the Aegean (CMRE) Ermoupolis, Syros, Greece La Spezia, Italy dzissis@aegean.gr paolo.braca@cmre.nato.int Figure 1: Visual representation of the data transformation flow. ABSTRACT potential congestion buildup and where cargoes should be rerouted Ships are often considered as the backbone of the global economy. to. Herein, we analyze a large-scale high-resolution mobility data A fundamental unresolved problem is how to best operate fleets, set of more than 7,000 container ships, collected over an extended given a sudden increase in demand, such as that reported following period of 36 months, covering the entire globe, so as to measure the first months of the COVID19 pandemic. Advancing our knowl- quantitatively the effects of the pandemic post-hoc on the supply edge of the supply chain’s delicate equilibrium between demand chain. To further understand these fine-grained mobility patterns, and supply, requires analyzing huge amounts of ship-related posi- we introduce a mobility model for calculating ship presence times tional data, thus revealing which areas should be avoided due to (or waiting times) at a global scale. We then reveal the congestion points, which strongly correlate with port waiting areas and anchor- ∗ These authors contributed equally to this work † Corresponding author ages. We analyse the data to reveal how times at port areas were affected by the rising number of ships waiting to load or unload Copyright © 2024 for this paper by its authors. Use permitted under Creative Commons cargo. Following this, we transform this data into a ‘port to port’ License Attribution 4.0 International (CC BY 4.0). Published in the Proceedings of the Workshops of the EDBT/ICDT 2024 Joint Conference (March 25-28, 2024), Paestum, graph mapping the international flow of containerised trade. We Italy. CEUR ceur-ws.org Workshop ISSN 1613-0073 Proceedings apply methods of graph theory, complex networks, and multivari- the hidden patterns is intrinsic to trajectory data mining, and the ate statistics to unravel the hidden relationships between global seminal work in [28] covers the field in detail. This growth has been maritime structure and ship time distribution. This analysis is novel boosted also by the availability of data from large sensor networks, in respect to the size of the data analyzed, the algorithmic approach such as the AIS, which has provided researchers with enormous vol- and the impact of the results which reveal some affinities between umes of information for the study of maritime transportation and pre-COVID and post-COVID shipping patterns. the maritime industry in general. According to the authors of [10], one of the main scientific topics discussed in the maritime literature is indeed the applications of big data techniques to AIS. Recently, 1 INTRODUCTION mostly due to the COVID-19 pandemic, but also to other events, The supply chain is a complex network involving suppliers, trucks, such as the Suez canal blockage in 2021 and the Red Sea crisis in distribution centers, warehouses, logistics centers and vectors, 2024, the maritime transportation has been often disrupted [19, 23] working together in concert to deliver goods to the customers. and several studies were conducted to measure effects of this disrup- Today, this complex network is distributed all around the world. tion [15, 17]. The strong academic interest to study large mobility With more than 80% of global trade by volume and up to 70% of its data at scale combined with necessity for fast decision making dur- value being carried by the shipping industry, vessels can be seen ing the pandemic has accelerated advancements in the field [29]. as the backbone of the global supply chain. Thus, shipping can be In this work, we use big data analytics and graph analysis to bet- viewed as a barometer for the global economic climate, and any ter understand the disruption in the supply chain. The utilisation of reduction in activity is expected to have a cascading effect on the big data in AIS analysis is an area of research that has received a lot global economy. On the other hand, ports are located at the nexus of attention [3, 24, 30] recently. For instance, in [22], authors evalu- of the supply chain and connect global and regional actors. ate the performance of clustering algorithms for route modelling The COVID-19 pandemic and containment measures had a sig- on a full year global AIS dataset, and in [16], the KDE technique is nificant impact on global maritime traffic in the short term. Several adapted to map-reduce paradigm to compute seaports’ extended ar- studies have attempted to quantify these effects and assess the size eas of operations from AIS data. Other examples are [20], where an of this reduction. However, after the initial shock of the pandemic, image analysis on density maps to detect traffic flows is introduced, when customer demand rebounded, vessels stacked up high with as well as [27], in which authors introduce trip semantic objects and containers could be seen anchored outside many of the world’s the use of density based clustering to identify clusters of way-points major ports. and stops. In a complementary to this work approach, authors in [4] Starting from the idea that shipping traffic data can be used build voyage graph feature time-series (VGT) to study their evolu- to assess the effects of restrictive measures on the global supply tion from a time-series perspective. In this work, we quantify and chain, in this work we focus on revealing the major delay points allocate to ports the effects of each vessel slow-down in range by in the maritime global supply network, as well as attempting to introducing the waiting and approaching time indicators. We study answer the question if these correlate with major port locations. their evolution over time to understand which ports are affected We also explore if congestion at ports is simply related to a sudden the most. increase in ship traffic and evidently quantify the increase in delay Graph theory, and its widely known extension known as com- times. Along these lines, we attempt to reveal how the pandemic plex networks, can be employed to characterize (port) nodes by has affected ports over an extended period of time and attempt to providing a hierarchy of centrality/accessibility in the container reveal the trajectory of recovery. shipping network. Early applications provided global-network mea- The main technical challenge is that of a big-data mining task of sures of connectivity [11, 12, 26], as well as a cartography of degree transforming huge amounts of geospatial data–as collected from or betweenness centrality [9]. Research on the relationship between vessels using the Automatic Identification System (AIS)–into a de- centrality and other port performance indicators remains relatively scriptive and compact data model, that can be used for identifying scarce in the literature [13, 14, 25], usually confirming the strong the underlying relationships or patterns. In our case, the patterns correlation between degree centrality (i.e., number of connections are those of normal port to port traffic connections. Our approach to other ports) and weighted degree (i.e., total traffic in twenty-foot relies on data transformations and distributed raster-based analyt- equivalent units [TEUs]). Thus, in the present study, we innovate ics as a first step to reduce the size of the data, followed by graph by applying a statistical analysis of traffic, centrality, and time in- analysis to reveal the hidden patterns in the data. dicators. It complements the work of Ducruet and Itoh [8] on the The main methodological contribution of this work is showing statistical relationships at stake between ship time, port centrality, that mobility data can be processed to shed light on the temporal and port traffic by focusing on a specific event and its supply chain and spatial characteristics of the supply chain as a network at the consequences. A recent review of the field [7] also showed that global scale. To the best of our knowledge, no major study has within a corpus of 212 papers about shipping networks published attempted to analyze such a large dataset with this aim before. between 2007 and 2022, nearly 20% concerned the topics of crisis and vulnerability, i.e., the second largest category after "network 1.1 Related work and contribution structure". Over the last years, there has been an exponential growth of scien- tific publications related to maritime traffic analysis involving big data analytics and/or novel Artificial Intelligence (AI) techniques. For instance, the analysis of vessel mobility data to understand 2 METHODOLOGY segment. We annotate as idle any segments whose average speed is less than 2 knots, and finally we sum up the total time of idle 2.1 Automatic Identification System segments for each cell and month. High values of idle times are AIS was originally designed as a collision avoidance system for indicators of choke points for shipping traffic, and the computation ships. Since 2002, the International Maritime Organisation (IMO) of idle time rasters on a monthly basis allows us to characterize has made compulsory for all vessels with a tonnage including how the distribution changes over the three-year period consid- and above 300 gross tonnage to be equipped with a class-A AIS ered. Anticipating the results, we observe that high idle time cells transceiver. At its core, each AIS transceiver sends and receives typically appear near major container ports and canals. positional reports (i.e., types 1, 2, 3 and 18) every few seconds Connecting waiting areas with ports. To further investigate via VHF. The messages contain information about each vessel’s this behavior and explicitly connect cells of high cumulative idle identity, location, course and speed. Since 2006, the lower-power time with ports, we performed a nearest neighbor analysis [2] (and lower-cost) class-B transceiver was introduced, allowing also to assign each cell to its nearest port. Then, if the cell is located smaller vessels to use the AIS, even if with lower performance and within a 100 km range of any top-50 ports 1 (in terms of annual priority than commercial fleets, which operate strictly on class- reported volume [1]) the cell is reassigned to its closest top-50 port. A transceivers [21]. The transmission rate of AIS ranges from 2 Again, anticipating a bit the results of our analysis, we notice an seconds, for fast moving vessels or maneuvering vessels equipped increase of cumulative idle time around major ports both in terms with a class-A transponder, up to 3 minutes for anchored or moored of cumulative values and number of cells where this happens. vessels. Measuring in-port and approaching time. The previous steps For this study, we make use of AIS positional reports of container leave us with the congestion epicenters near major ports, where ships traveling across the globe for the years 2019, 2020 and 2021. vessels wait to enter the port. The epicenters are located, in most cases, near the ports. However, as congestion increases, the wait- 2.2 Data transformation ing areas expand vastly following different patterns with respect To unravel the hidden information of global supply chain perfor- to topology and other local characteristics. It is also possible that mance from raw AIS messages, we employ a multi-step sequential a non-negligible number of vessels sheltered themselves in these data mining process. Our main goal is understanding if supply chain areas and never entered the closest major port. To confirm or reject disruption is measurable and correlated with port activities. Then, this hypothesis, we used accurate information about the end of we also investigate what are the intrinsic characteristics of these each itinerary. The AIS protocol supports messages that include ports, to understand if they can be possibly used in a predictive information about the destination port, but unfortunately it cannot fashion as indicators of future disruptions, so that fleets can be be considered as a reliable source of information, as it is manually rerouted suitably. In our case study, the effects of a disruption in entered by the crew, without following a specific standard, mak- global supply chain are not known beforehand. We introduce a pro- ing it thus extremely prone to errors. To tackle this problem, we cess (Fig. 1) to infer a global supply chain network graph from AIS performed a retrospective analysis on the data to identify the ports mobility data. Then, we apply advanced graph analytics to identify of origin and destination for each trip, and we calculated the exact port typologies and changes over its connections to measure effects time of approaching 300 km to destination, as well as the exact time of disruptions over a three-year period. of each vessel entering the port across all itineraries that reach any Data cleaning & conditioning The first step in the process is of the top-50 ports. Then, we calculate for each itinerary the total a cleaning task to ensure that records comply with protocol stan- time spent within a 300 km radius and the time spent in the port. dards and reject records with missing values. Then, we apply a The 300 km radius ensures that we account for any intentional or geo-fencing technique to select records located within port areas, unintentional delay that may occur for any vessel before it reaches and exclude them from the identification of waiting areas part of its final destination. This radius is selected so that all waiting areas the analysis. To facilitate the numerical calculations, all positional of the first part of our analysis are included. The time in port reflects reports are re-projected into the Web Mercator (EPSG:3857) coordi- the operational time of a vessel calling a port and it captures all nate system. time required to moor at berth and perform all kinds of loading and Raster-based analysis. To identify waiting areas from AIS mes- unloading operations and exit the port. sages, we use a raster-based analysis. We first define the raster Defining the waiting time network. Maritime flows can be characteristics, such as the shape and size of its cells. For the analy- modeled as a graphical structure G, where the ports (𝑣) are the sis performed in this work, the raster consisted of square-shaped nodes (or vertices), which are connected by inter-port connections cells of a 9.7 km side length, each one of them covering an area of (𝑒) as links (or edges), so that G = (𝑣, 𝑒) [6]. The connections among approximately 100 km2 on average with respect to the projection ports are in general not known a priori and can change over time, systems’ distortion. Then, we assign the AIS messages to the grid but in principle they can be learned by inspection of the AIS data by cells, by splitting each trajectory into segments that match the grid looking at vessels navigating from one port to another. Each vertex definition (i.e., each segment is allocated to exactly one cell and 𝑣 in the graph G stores static features and summary statistics of the it is annotated with the cell’s id). Each segment consists of either port’s traffic flow strength that corresponds to the graph weights two consecutive AIS messages or a grid intersection point and an and they are calculated on quarterly basis. The static features are the AIS message,where the location and timestamp of the intersection point are interpolated assuming constant speed. Then, we calculate 1We consider the top-50 ports to be representative of the whole port system in their the average speed required for a vessel to cover the distance of each proximity. port identifier and country each port belongs to, while the summary top of the diagram. Interestingly, we can see the fluctuation in the statistics measure the number of vessels calling the port, as well positions, with ports switching their ranking throughout the year. as their aggregated maximum capacity. Each link 𝑒 ∈ G consists To understand the characteristics of the ports and areas of delay, of the pair of ports identifiers it connects, as well the number of we move onto the second part of the analysis, which makes use of voyages on this connection and their cumulative maximum TEU graph theory and PCA. capacity and aggregated time indicators. Those statistics are also calculated in correspondence to nodes on a quarterly basis. Graph analysis To assess the level of disruption on port con- 3.1 Single and multivariate linkage analysis nections, we rely on quarterly created summary statistics for nodes We apply PCA to two distinct datasets: 1) based on static port char- (𝑣) from the previous step, and we calculate the differences for the acteristics in Q4 2019; and 2) based on absolute changes of these total and the in port time between the last quarter (Q4) of 2019 characteristics between Q4 2019 and Q1 2020. Both datasets include and the first quarter (Q1) of 2020. We apply linear transformations time evolution as a means of checking its affinity with other vari- to summary statistics to define supply chain port characteristics ables, which are its potential determinants. The two PCAs provided such as the number of vessels calls (frequency) and total vessel interesting results, with 72.1% of variance contained in the three traffic (frequency × vessel capacity in TEU). We complement our first components for static variables (with eigenvalues > 1), and dataset with graph-theoretical indices calculated for all network 76.0% for the first four components for dynamic variables (with nodes and both quarters, namely the degree centrality (number of eigenvalues > 1). Figure 3 represents the distribution of variables shipping links), betweenness centrality (number of occurrences on along the two first components for each dataset (left, static; right, shortest paths in the graph), and inverse clustering coefficient (local dynamic). Interestingly, worsening time is opposed to traffic and hub power). The average port time in Q4 2019 will also be used centrality level/growth in the two figures. This is even truer for dy- as a pre-existing characteristic. We apply a Principal Components namics, where calls (trip_dif) and traffic (teu_dif) are more directly Analysis (PCA) to all nodes to the (Q4) of 2019 quarter and quar- opposed to port time evolution. Another difference between the two terly calculated residuals to reveal the hidden trends at stake in the PCAs is the opposition between connectivity changes and traffic shipping traffic data. PCA is a statistical method serving to unravel changes along the second component (vertical axis) for dynamics. a limited number of unobserved (latent) variables among a set of It means that although growing ports in general witnessed reduced observed, correlated variables [5]. Such unobserved variables, often port time, those with growing connectivity tended to increase port called principal components or “factors”, constitute the basis of a time, contrary to ports increasing traffic. Lastly, the static analysis clustering analysis that will distribute observations (port nodes) shows that ports with longer times in Q4 2019 were also the ones among distinct groups. The clusters, i.e. groups of ports of similar increasing port time in Q1 2020 (vertical axis). operational behaviour (see bottom right legend in Figure 4), are The situation of each port in the observed trends is revealed then confronted to initial variables to best describe their trends and by means of a hierarchical clustering analysis, which is applied to characteristics. The next step is to illustrate the typology by means the main components of each PCA, to produce a typology. This of a single linkage analysis. This method serves to highlight the is combined with a single linkage analysis, to test whether the main hubs and their “nodal regions” by keeping only the largest obtained types have a specific position in the network’s backbone. traffic flow link of each port in the graph. Finally, a multiple re- The dynamics-based typology provides the picture of world ports gression looks at the determinants of time evolution based on port reported in Fig. 4. It considers absolute changes of port character- characteristics. istics as for the second PCA, and the single linkage analysis is based on Q1 2020. The most impacted category (yellow) is marked by drastic traffic decline, slight reduction of centrality, and the strongest increase of total and in-port time. It includes a vast ma- 3 RESULTS jority of gateway ports (Le Havre, Constanta, Koper, Alexandria, Areas where ships “wait” evidently depict problematic spots in the Liverpool, Felixstowe, Zeebrugge, Fos, Los Angeles, Long Beach, supply chain. In order to reveal the spatiotemporal characteristics of Tianjin, Lianyungang, Kobe, Ho Chi Minh, Manila) as well as Hong delay areas globally, we first define and quantify areas where ships Kong and Port Klang. Except from the latter two ports, these gate- are idle for long periods, as these can be an indicator of disruptions ways have, in general, a limited role in the architecture of nodal in the supply and demand balance [17]. Our analysis focused on first regions, due to their specialization in import/export cargoes. An- understanding if these areas overlapped with specific port areas other category has lost similar amounts of traffic on average (red) and then to further understand if these had specific characteristics. but such ports slightly increased their centrality. Several of them As a first result, which may have been expected, we confirmed are large hub ports polarizing their respective nodal region, the that waiting areas are close to port locations. All top-30 locations largest being, like in the previous figure, Singapore, Busan, and are within 80 nautical miles range from ports, and we can assume Rotterdam. Like for the other categories (green, blue), these ports reasonably that ships stationing in these areas are waiting to enter experienced a slight increase of total port time and small decrease the port. This result is in line with reports and papers reporting the of in-port time. While they also lost traffic, the secondary hubs increase turn around in port areas [18]. (green) gained enormous centrality in Q1 2020, contrary to what In Fig.2, we illustrate a Sankey Diagram ranking ports according we can call second-tier hubs (blue), which have the opposite profile. to the measured total idle time over the last time period of the There is no apparent geographic or functional logic in those two analysis, where ports with longer waiting times are located at the categories, which are disseminated across regions and contain both Figure 2: Visual representation in terms of total idle time for the top-10 ports of each month across 2021 year. Line width represents the value of total idle time. AMB:Ambarli, ANT:Antwerp, BRE:BremerHaven, BUS:Busan, CAP:Cap Town, DAL:Dalian, DAR:Dar ES Salaam, HAM:Hamburg, HON:Hong Kong, JAK:Jakarta, JEB:JEBEL ALI, LOM:LOME, LOS:Los Angeles, MAN: Manzanillo, NIG: Nigbo, OAK:Oakland, PIR: Piraeus, POR: Port Lang, Rot: Rotterdam (blue: Waalhaven, orange:Maasvlakte), SAV: Savannah, SHA: Shanghai, SIN: Singapore, VAL: Valencia. gateway ports and transshipment ports. The loss of centrality (blue) centrality indicators (cf. Principal Component Analysiss (PCAs)), is, still, relatively common to European ports while the increase of it expresses a specific dimension of port connectivity, namely the centrality (green) is better found in Asia. ability to polarize neighboring, or adjacently connected, ports. A counter-intuitive result is the negative effect of city size (pop- 3.1.1 What determined port time changes in 2020? A multiple re- ulation) on time evolution in both models, as the inclusion of this gression analysis is applied in two steps, each being a model focus- variable was meant to test the role of potential congestion played ing on a distinct independent variable: total port time difference by cities on port operations, in terms of lack of space and density. (model 1) and in-port time difference (model 2), as shown in Ta- This can be explained by our focus on the top of the port hierarchy, ble 1. As a matter of fact, among the selected dependent variables, where most ports are in fact major metropolitan areas. Another only two have a statistically significant effect. It is the case of in- commonality between the two models is the negative influence of port time in model 1 (0.05 significant), which increased total port total port time and the positive influence of in-port time. In-port time difference between Q4 2019 and Q1 2020. The other case is the time is a component of total port time, but it better represents the regional dummy Africa in model 2, which increased in-port time dif- core activity of the port, as it is the closest to the length taken by ference. Despite the low significance of other dependent variables, terminal operations. This crucial component of the whole transport some of them may be discussed according to the direction of their chain, if prolonged, will inevitably have strong consequences on effect on time evolution. Among the ones that deserve attention, the rest of the chain, as seen with its positive impact on total time inverse clustering coefficient stands out as it has the same, negative difference (slowdown, queuing), which includes the water vicinity effect on time evolution, and is near-to-significant in both models. of the port (e.g. port entrance, access channel). Thus, ports with an It means that ports ensuring stronger hub functions before the already high in-time (turnaround time) have witnessed worsening crisis have witnessed lesser congestion and, even, more fluid cargo operations (prolonged times) during the COVID-19 crisis. At the transfers. Such a result is in line with the single linkage analyses, contrary, total port time had a negative effect on time evolution showing that pivotal hubs ensure and maintain their domination in both models. While such a result may seem to contradict the towards other ports, often within a certain geographic radius (nodal former, it should be understood in the light of other port variables regions). It also confirms the work of Ducruet and Itoh [8] about the in each model. In model 1, the negative effect of total port time on determinants of ship turnaround times on a long period. Although total port time difference goes along with a negative effect of port this measure is very much correlated with port traffic and other size (calls, TEUs), meaning that large, busy ports in Q4 2019 (but (a) (b) Figure 3: Principal component analyses Table 1: Determinants of port time evolution. Model 1 Model 2 total time difference in-port time difference Estimate Std. err. 𝑡-value Pr(> |𝑡 |) Estimate Std. err. 𝑡-value Pr(> |𝑡 |) (Intercept) 1.409 00 1.201 00 1.173 0.2440 −0.720 11 0.524 60 −1.373 0.1735 Population −0.024 69 0.035 61 −0.694 0.4899 −0.008 62 0.015 55 −0.554 0.5809 Calls −0.093 89 0.232 51 −0.404 0.6874 0.164 42 0.101 56 1.619 0.1092 TEUs −0.127 66 0.141 70 −0.901 0.3702 0.024 27 0.061 89 0.392 0.6960 Distance −0.037 87 0.088 34 −0.429 0.6692 0.038 61 0.038 59 1.000 0.3200 Betweenness Centrality 0.023 93 0.079 31 0.302 0.7636 0.010 52 0.034 64 0.304 0.7621 Degree Centrality 0.460 94 0.332 03 1.388 0.1687 −0.238 50 0.145 03 −1.645 0.1038 I.C.C.† −0.048 19 0.031 99 −1.506 0.1357 −0.016 58 0.013 97 −1.186 0.2388 Total time −0.167 66 0.135 24 −1.240 0.2185 −0.083 76 0.059 07 −1.418 0.1599 In-port time 0.581 33 0.263 06 2.210 0.0298* 0.084 71 0.114 91 0.737 0.4630 Africa −0.025 91 0.699 50 −0.037 0.9705 0.525 12 0.305 55 1.719 0.0893 Americas 0.180 97 0.664 12 0.273 0.7859 0.147 99 0.290 09 0.510 0.6113 Asia 0.405 55 0.665 50 0.609 0.5439 0.194 31 0.290 69 0.668 0.5057 Europe 0.087 54 0.662 63 0.132 0.8952 0.264 30 0.289 44 0.913 0.3638 † Inverse clustering coefficient not necessarily the most central, as seen with the positive influence a negative influence of degree centrality (numerous connections). of betweenness and degree) had more chance to perform better in This means that hub ports with a relatively lower size managed to times of crisis. In model 2, the same negative effect is associated improve their core operations in the advent of the crisis. with a positive influence of port size (worsening in-port time), and Figure 4: Single linkage analysis and port typology – Q1 2020 and port/time evolution 4 CONCLUSION ACKNOWLEDGMENTS Based on geospatial data mining approaches, in this paper we pre- This work was partially supported by MarineTraffic. This research sented a methodology to determine the misbalances in the global is supported by European Union’s Horizon 2020 research and inno- supply and demand equilibrium as captured through ship move- vation programme under grant agreement No 101092749, project ments. The presented approach is capable, firstly, of detecting and Critical Action Planning over Extreme-Scale Data (CREXDATA). defining the areas of potential delays, which in most cases overlap The work of L. M. Millefiori and P. Braca is supported by NATO with the main port areas, and secondly to determine the specific Allied Command Transformation (ACT) via project “Data Knowl- characteristics of these ports. Our approach relies on methods of edge Operational Effectiveness” (DKOE). The contribution of César raster based analysis, graph theory and complex networks analy- Ducruet is supported by the French National Research Agency sis. Future work will be focused on applying additional methods (ANR) through the research project No. ANR-22-CE22-0002 "Mar- from the field of graph analysis and complex networks to similar itime Globalization, Network Externalities, and Transport Impacs datasets. on Cities" (MAGNETICS). REFERENCES Transportation Review 95 (2016), 326–340. [1] World Shipping Council. 2020. The Top 50 Container Ports. https://www. [26] Deng Wei-Bing, Guo Long, Li Wei, and Cai Xu. 2009. 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