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**High Performance Discovery from Time Series Streams**

Dennis Shasha Joint work with Yunyue Zhu Courant Institute, New York University

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**Overall Outline Data mining – both classical and activist**

Algorithmic tools for time series Surprise.

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Goal of this work Time series are important in so many applications – biology, medicine, finance, music, physics, … A few fundamental operations occur all the time: burst detection, correlation, pattern matching. Do them fast to make data exploration faster, real time, and more fun. Extend functionality for music and science.

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**StatStream (VLDB,2002): Example**

Stock prices streams The New York Stock Exchange (NYSE) 50,000 securities (streams); 100,000 ticks (trade and quote) Pairs Trading, a.k.a. Correlation Trading Query:“which pairs of stocks were correlated with a value of over 0.9 for the last three hours?” XYZ and ABC have been correlated with a correlation of 0.95 for the last three hours. Now XYZ and ABC become less correlated as XYZ goes up and ABC goes down. They should converge back later. I will sell XYZ and buy ABC …

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**Online Detection of High Correlation**

Given tens of thousands of high speed time series data streams, to detect high-value correlation, including synchronized and time-lagged, over sliding windows in real time. Real time high update frequency of the data stream fixed response time, online Correlated!

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**Online Detection of High Correlation**

Given tens of thousands of high speed time series data streams, to detect high-value correlation, including synchronized and time-lagged, over sliding windows in real time. Real time high update frequency of the data stream fixed response time, online

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**Online Detection of High Correlation**

Given tens of thousands of high speed time series data streams, to detect high-value correlation, including synchronized and time-lagged, over sliding windows in real time. Real time high update frequency of the data stream fixed response time, online Correlated!

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**StatStream: Algorithm**

Naive algorithm N : number of streams w : size of sliding window space O(N) and time O(N2w) VS space O(N2) and time O(N2) . Suppose that the streams are updated every second. With a Pentium 4 PC, the exact computing method can only monitor 700 streams with a delay of 2 minutes. Our Approach Use Discrete Fourier Transform to approximate correlation Use grid structure to filter out unlikely pairs Our approach can monitor 10,000 streams with a delay of 2 minutes.

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**StatStream: Stream synoptic data structure**

Three level time interval hierarchy Time point, Basic window, Sliding window Basic window (the key to our technique) The computation for basic window i must finish by the end of the basic window i+1 The basic window time is the system response time. Digests Basic window digests: sum DFT coefs Sliding window Basic window Time point

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**StatStream: Stream synoptic data structure**

Three level time interval hierarchy Time point, Basic window, Sliding window Basic window (the key to our technique) The computation for basic window i must finish by the end of the basic window i+1 The basic window time is the system response time. Digests Basic window digests: sum DFT coefs Sliding window Basic window Time point Basic window digests: sum DFT coefs

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**StatStream: Stream synoptic data structure**

Three level time interval hierarchy Time point, Basic window, Sliding window Basic window (the key to our technique) The computation for basic window i must finish by the end of the basic window i+1 The basic window time is the system response time. Digests Basic window digests: sum DFT coefs Sliding window Basic window Time point Basic window digests: sum DFT coefs Sliding window digests: sum DFT coefs

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**StatStream: Stream synoptic data structure**

Three level time interval hierarchy Time point, Basic window, Sliding window Basic window (the key to our technique) The computation for basic window i must finish by the end of the basic window i+1 The basic window time is the system response time. Digests Basic window digests: sum DFT coefs Sliding window Basic window Time point Basic window digests: sum DFT coefs Sliding window digests: sum DFT coefs

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**StatStream: Stream synoptic data structure**

Three level time interval hierarchy Time point, Basic window, Sliding window Basic window (the key to our technique) The computation for basic window i must finish by the end of the basic window i+1 The basic window time is the system response time. Digests Basic window digests: sum DFT coefs Basic window digests: sum DFT coefs Basic window digests: sum DFT coefs Time point Basic window Sliding window

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**Synchronized Correlation Uses Basic Windows**

Inner-product of aligned basic windows Stream x Stream y Basic window Sliding window Inner-product within a sliding window is the sum of the inner-products in all the basic windows in the sliding window.

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**Approximate Synchronized Correlation**

Approximate with an orthogonal function family (e.g. DFT) x x x x x x x x8 f1(1) f1(2) f1(3) f1(4) f1(5) f1(6) f1(7) f1(8) f2(1) f2(2) f2(3) f2(4) f2(5) f2(6) f2(7) f2(8) f3(1) f3(2) f3(3) f3(4) f3(5) f3(6) f3(7) f3(8)

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**Approximate Synchronized Correlation**

Approximate with an orthogonal function family (e.g. DFT) x x x x x x x x8

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**Approximate Synchronized Correlation**

Approximate with an orthogonal function family (e.g. DFT) x x x x x x x x8 y y y y y y y y8

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**Approximate Synchronized Correlation**

Approximate with an orthogonal function family (e.g. DFT) Inner product of the time series Inner product of the digests The time and space complexity is reduced from O(b) to O(n). b : size of basic window n : size of the digests (n<<b) e.g. 120 time points reduce to 4 digests x x x x x x x x8 y y y y y y y y8

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**Approximate lagged Correlation**

Inner-product with unaligned windows sliding window The time complexity is reduced from O(b) to O(n2) , as opposed to O(n) for synchronized correlation. Reason: terms for different frequencies are non-zero in the lagged case.

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**Grid Structure(to avoid checking all pairs)**

The DFT coefficients yields a vector. High correlation => closeness in the vector space We can use a grid structure and look in the neighborhood, this will return a super set of highly correlated pairs. x

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**Empirical Study : Speed**

Our algorithm is parallelizable.

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**Empirical Study: Precision**

Approximation errors Larger size of digests, larger size of sliding window and smaller size of basic window give better approximation The approximation errors are small for the stock data.

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**Sketches : Random Projection**

Correlation between time series of the returns of stock Since most stock price time series are close to random walks, their return time series are close to white noise DFT/DWT can’t capture approximate white noise series because there is no clear trend (too many frequency components). Solution : Sketches (a form of random landmark) Sketches pool: matrix of random variables drawn from stable distribution Sketches : The random projection of all time series to lower dimensions by multiplication with the same matrix The Euclidean distance (correlation) between time series is approximated by the distance between their sketches with a probabilistic guarantee.

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Burst Detection

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**Burst Detection: Applications**

Discovering intervals with unusually large numbers of events. In astrophysics, the sky is constantly observed for high-energy particles. When a particular astrophysical event happens, a shower of high-energy particles arrives in addition to the background noise. Might last milliseconds or days… In telecommunications, if the number of packages lost within a certain time period exceeds some threshold, it might indicate some network anomaly. Exact duration is unknown. In finance, stocks with unusual high trading volumes should attract the notice of traders (or perhaps regulators).

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**Bursts across different window sizes in Gamma Rays**

Challenge : to discover not only the time of the burst, but also the duration of the burst.

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**Elastic Burst Detection: Problem Statement**

Problem: Given a time series of positive numbers x1, x2,..., xn, and a threshold function f(w), w=1,2,...,n, find the subsequences of any size such that their sums are above the thresholds: all 0<w<n, 0<m<n-w, such that xm+ xm+1+…+ xm+w-1 ≥ f(w) Brute force search : O(n^2) time Our shifted wavelet tree (SWT): O(n+k) time. k is the size of the output, i.e. the number of windows with bursts

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**Burst Detection: Data Structure and Algorithm**

Define threshold for node for size 2k to be threshold for window of size 1+ 2k-1

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**Burst Detection: Example**

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**Burst Detection: Example**

False Alarm True Alarm

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**False Alarms (requires work, but no errors)**

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**Empirical Study : Gamma Ray Burst**

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**Extension to other aggregates**

SWT can be used for any aggregate that is monotonic SUM, COUNT and MAX are monotonically increasing the alarm threshold is aggregate<threshold MIN is monotonically decreasing Spread =MAX-MIN Application in Finance Stock with burst of trading or quote(bid/ask) volume (Hammer!) Stock prices with high spread

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**Empirical Study : Stock Price Spread Burst**

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**Extension to high dimensions**

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**Elastic Burst in two dimensions**

Population Distribution in the US

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**How to find the threshold for Elastic Burst?**

Suppose that the moving sum of a time series is a random variable from a normal distribution. Let the number of bursts in the time series within sliding window size w be So(w) and its expectation be Se(w). Se(w) can be computed from the historical data. Given a threshold probability p, we set the threshold of burst f(w) for window size w such that Pr[So(w) ≥ f(w)] ≤p.

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**Find threshold for Elastic Bursts**

Φ(x) is the normal cdf, so symmetric around 0: Therefore Φ(x) p x Φ-1(p)

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Summary Able to detect bursts of many different durations in essentially linear time. Can be used both for time series and for spatial searching. Can specify thresholds either with absolute numbers or with probability of hit. Algorithm is simple to implement and has low constants (code is available). Ok, it’s embarrassingly simple.

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**With a Little Help From My Warped Correlation**

Karen’s humming Match: Dennis’s humming Match: “What would you do if I sang out of tune?" Yunyue’s humming Match:

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**Related Work in Query by Humming**

Traditional method: String Matching [Ghias et. al. 95, McNab et.al. 97,Uitdenbgerd and Zobel 99] Music represented by string of pitch directions: U, D, S (degenerated interval) Hum query is segmented to discrete notes, then string of pitch directions Edit Distance between hum query and music score Problem Very hard to segment the hum query Partial solution: users are asked to hum articulately New Method : matching directly from audio [Mazzoni and Dannenberg 00] slowed down by DTW

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**Time Series Representation of Query**

Segment this! An example hum query Note segmentation is hard!

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**How to deal with poor hum queries?**

No absolute pitch Solution: the average pitch is subtracted Incorrect tempo Solution: Uniform Time Warping Inaccurate pitch intervals Solution: return the k-nearest neighbors Local timing variations Solution: Dynamic Time Warping

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Dynamic Time Warping Euclidean distance: sum of point-by-point distance DTW distance: allowing stretching or squeezing the time axis locally

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**Envelope Transform using Piecewise Aggregate Approximation(PAA) [Keogh VLDB 02]**

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**Envelope Transform using Piecewise Aggregate Approximation(PAA)**

Advantage of tighter envelopes Still no false negatives, and fewer false positives

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**Container Invariant Envelope Transform**

Container-invariant A transformation T for envelope such that Theorem: if a transformation is Container-invariant and Lower-bounding, then the distance between transformed times series x and transformed envelope of y lower bound their DTW distance. Feature Space

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The Vision Ability to match time series quickly may open up entire new application areas, e.g. fast reaction to external events, music by humming and so on. Main problems: accuracy, excessive specification. Reference (advert): High Performance Discovery in Time Series (Springer 2004)

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Elastic Burst Detection: Applications Discovering intervals with an unusually large numbers of events. –In astrophysics, the sky is constantly observed.

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