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Causal Dynamic Time Lag (CDT)Applications to Space Weather

Mandar Chandorkar

1Multiscale DynamicsCWI, Amsterdam

2TAU Research UnitINRIA, Paris-Saclay

25 September 2018

Mandar Chandorkar (CWI & INRIA) Causal Dynamic Time Lag (CDT) 25 September 2018 1 / 47

Outline

1 MotivationSpace WeatherFinancial Markets

2 Causality in Time SeriesConceptsExisting Research

3 Problem & ModelDescriptionProposed Solution

4 ApplicationsBenchmarksProblem IProblem IIProblem III

Outline

Figure: The Sun-Earth system

Figure: Effects of Solar Disturbances

Outline

Figure: Yield curves and Recessions in the U.S. Economy

Outline

Granger Causality

1 The cause happens prior to its effect.

2 The cause has unique information about the future values of its effect.

Figure: By BiObserver - Own work, CC BY-SA 3.0,https://commons.wikimedia.org/w/index.php?curid=33470670

Outline

Nonparametric learning of time-lag

[Zhou and Sornette, 2006] formulated the problem as minimisation of themismatch between two time series.Inputs: X (t),Y (t), two time series.Learn: A mapping φ(t1) = t2 which minimises

ε(tt , t2) = |X (t1)− Y (t2)| (1)

Figure: An example of energy landscape EX ,Y given by (1) for two noisy timeseries and the corresponding optimal path wandering at the bottom of the valleysimilarly to a river. This optimal path defines the mapping t1→ t2 = φ(t1).

Outline

Causal Dynamic Time-lag Inference (CDT)

Given an input (cause) and output (effect) time series, predict

Magnitude of output signal. (What/How much)

When the effect will be observed in the output signal (When)

CDT: Formal Definition

Input Signal (Cause)

t ∈ R+

x(t) ∈ X

Output Signal (Effect)

f : X → Rg : X → R+

∆(t) = g [x(t)]

y(t + ∆(t)) = f [x(t)]

Outline

Model Specification

Input Patterns x(t)

Causal Time WindowLower Limit: ` ∈ N ∪ 0Upper Limit: `+ h : h ∈ N

Targets y(t + `), · · · , y(t + `+ h − 1)

Model OutputsPredictions y(t + `), · · · , y(t + `+ h − 1)Time Lag Probabilities p(t + `), · · · , p(t + `+ h − 1)

Training: Motivation

Balance two incentives

1 Generate accurate predictions for time windowy(t + `), · · · , y(t + `+ h − 1)

2 Learn time lag structure according to some intuition.

Training Loss

L(y (1:M), y (1:M), p(1:M)) =λ1∑i ,m

(m)i − y

(m)i )2(1 + p

(m)i )

λ2J (y (1:M), y (1:M), p(1:M))

Training Loss Contd.

The term J (y (1:M), y (1:M), p(1:M)) penalizes the predicted probabilitiesp(1:M), for deviation from some chosen target probability.

Target Probability

The target probability p for a time window [t + `, t + `+ h − 1] can becharacterized by:

Conjecture: Causal Time Lag

The lagged output y(t + i) which has greater predictability given x(t), is amore likely causal link.

Target Probability Contd.

1 The target probability distribution for the time lag is,p(m) = softmax((y (m) − y (m))2/T )

2 The term J (y (1:M), y (1:M), p(1:M)) can be computed as the Hellingerdistance between p and p.

Outline

Need for Benchmarks

1 Labelled data sets are small in size.

2 Most real world data sets dont have explicit labels for causal time lag.

Benchmark Problems

x(t + 1) = (1− β)x(t) +N (0, σ2)

y(t + ∆(t)) = α||x(t)||2

Problem I: Constant Lag

∆(t) = k

Problem II: Constant Velocity ||x(t)||2; Fixed Distance d

∆(t) = d/(α||x(t)||2)

Problem III: Constant Acceleration a; Fixed Distance d

∆(t) = (√α2||x(t)||4 − 2ad − α||x(t)||2)/a

Outline

Figure: Generated Data

Errors

Figure: Error in Output vs Error in Time Lag

Predictions

Figure: Test Set Time Series vs Predictions

Performance

Output

MAE: 8.602Pearson Corr: 0.964Spearman Corr: 0.999

Time Lag

MAE: 0Pearson Corr: N.ASpearman Corr: 1.0

Outline

Time Lags

Figure: Time Lags

Test Data Distribution

Figure: Output vs Time Lags

Predictions

Errors

Performance

Output

Time Lag

Outline

Time Lags

Figure: Time Lags

Test Data Distribution

Figure: Output vs Time Lags

Predictions

Errors

Performance

Output

Time Lag

MDG-TAU Team

Website http://mlspaceweather.org

References I

Zhou, W.-X. and Sornette, D. (2006).Non-parametric determination of real-time lag structure between twotime series: The optimal thermal causal path method withapplications to economic data.Journal of Macroeconomics, 28(1):195 – 224.Nonlinear Macroeconomic Dynamics.

causal dynamic time lag (cdt) - inria › ... › 11 › mandarchandorkar-mdg-tao-talk-sep… ·...

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