Event Detection and Characterization in Dynamic Graphs

Stony Brook University
Shebuti & Leman
Shebuti Rayana Leman Akoglu

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Shebuti & Leman Event Detection & Characterization in Dynamic Graphs
2

 Problem: Given a sequence of graphs,
Q1. Event detection: find time points at which
graph changes significantly
Q2. Characterization: find (top k) nodes / edges /
regions that change the most
3

 Main framework
 Compute graph similarity/distance scores
… … …
time
 Find unusual occurrences in time series
4

 Flow of Ensemble Approach
 Event Detection in Dynamic Graphs
Ensemble Algorithms
Eigen Behavior based Event Detection (EBED)
Probabilistic Approach (PTSAD)
SPIRIT
Consensus Method
Rank based
Score based
Results
Dataset 1: Challenge Network flow Data
Dataset 2: New York Times News Corpus
5

Event Detection
Consensus Rank Merging
•Rank based
•Inverse Rank
•Kemeny Young
•Score Based
•Unification
(avg, max)
•Mixture Model (avg,
max)
• Final Ensemble
(Inverse Rank)
Characterization
Shebuti & Leman Event Detection & Characterization in Dynamic Graphs 6

 Numerous algorithms for event detection
 Hard to decide which one will work well
for a specific data set
 Our Goal: design an ensemble approach which
might not give best result
but “better” than most base algorithms
 Challenges:
 Different scores/scales
 Different merging approaches
7

 Extract “typical behavior” (eigen-behavior) of
nodes/edges
 eigen-behavior ≡ principal eigen-vector
 Compare eigen-behavior over time
 Score the time ticks depending on
amount of change in behavior
from previous time tick.
 Mark the ones with high score as
anomalous.
T
N
Feature: Degree
8

Nodes
T
Features
(egonet)
Time
T
N
Feature:
degree
WW
past pattern
right
singular
vector
N



eigen-behavior at t eigen-behaviors
change-score
metric: Z = 1- uTr
9

 Individual nodes/edges time series with
distributions
 Poisson
 Zero-inflated Poisson
 Hurdle Process
▪ Hurdle Component: Bernoulli & Markov Chain
▪ Count Component: Zero-truncated Poisson
 Model Selection:
 AIC, log likelihood, Vuong’s test and log gain
 Find single-sided p-value as the probability of
observing a count as extreme as v [P(X ≥ v)]
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 Streaming Pattern dIscoveRy in multIple Time-series
(SPIRIT) [Papadimitriou et al. 2005]
 Discovers trends – whenever trend changes it
introduce new hidden variable & remove when not
needed
 Detects anomalous points in trends
 Nodes weights change in each step
 At a change point the node which has highest weight
is most anomalous
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Event Detection
Characterization

RankList2
ScoreList2
Consensus
RankList1
ScoreList1
Rank based Score based
•Inverse Rank
•Kemeny Young
[J. Kemeny 1959]
RankList3
ScoreList3
•Unification [Zimek et al. 2011]
-avg & max
•Mixture Model [Jing et al. 2006]
-avg & max
Final Ensemble: inverse rank
FinalRankList

 We were given a “Cyber Challenge Network”
from NGAS R&T Space Park
 Simulated cyber network traffic
 10 days activities
 125 hosts
 To-from information with timestamps
 Find “suspicious” events and the entities
associated with the corresponding events in
Challenge Network.
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Eigen-behaviors
Probabilistic Approach
SPIRIT
Z-score
1 – norm.
(sum
p-value)
projection
Time tick
Shebuti & Leman 16
Event Detection & Characterization in Dynamic Graphs
Feature:
Degree

Eigen-behaviors
at Time tick 376
SPIRIT
relative
activity
change
projection
weight
nodes
Shebuti & Leman 17
Event Detection & Characterization in Dynamic Graphs
normal.
|log(p)|

Average Precision Table (Feature: Degree)
Algorithm Sample rate (10 min)
Base
Algorithms
EBED 0.8333
PTSAD 0.5722
SPIRIT 0.7292
Consensus
Rank
Merging
Algorithms
Inverse Rank (1/R) 1.0000
Kemeny Young 0.8095
Unification (avg) 0.8056
Unification (max) 0.7255
Mixture model (avg) 0.1684
Mixture model (max) 0.1684
Final Ensemble (1/R) 0.8667

Average Precision Table for Node anomalies
Feature: Degree [Sample rate 10 min]
Algorithm Event at 376 Event at 1126
Base
Algorithms
EBED 1.0000 1.0000
PTSAD 1.0000 0.2500
SPIRIT 0.3026 0.0213
Consensus
Rank
Merging
Algorithms
Inverse Rank (1/R) 1.0000 0.5000
Kemeny Young 1.0000 0.2000
Unification (avg) 1.0000 1.0000
Unification (max) 0.8333 1.0000
Mixture model (avg) 1.0000 1.0000
Mixture model (max) 1.0000 1.0000
Final Ensemble (1/R) 1.0000 1.0000

 ~8 years (Jan 2000- July 2007) of published
articles of New York Times
 Graph links: Co-mention of named entities
(people, places, organization)
 Sample rate: 1 week
 No ground truth
 Big Events detected:
 January, 2001 – George W. Bush elected US president
 September 11, 2001 – Terrorist attack in WTC
 February 1, 2003 – Space Shuttle Columbia Disaster
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Feature:
Weighted
Degree
Eigen-behaviors
Columbia disaster
SPIRIT
2001 election
Z Score
1 – norm.
(sum
p-value)
projection
9/11 WTC attack

 Heterogeneous detectors
 different scores
 different effectiveness (depending on dataset)
 Ensemble for event detection on dynamic graphs
 Multiple consensus (merging) approaches
 two-phase consensus finding
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 Near-future: Robust consensus by automatically
selecting effective base algorithms
 Challenge: no ground truth
 Near-future: real-time detection
 Event detection under diverse data sources
(e.g., news media, social media, the Web, …)
 Challenges: different entity types,
different time granularity,
entity resolution
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srayana@cs.stonybrook.edu
http://www.cs.stonybrook.edu/~datalab/
Judge a man by his questions rather than his answers.
-Voltaire
Event Detection
Characterization
26

Event Detection and Characterization in Dynamic Graphs

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