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# Practical Time Series Analysis

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## What are Time Series?

- Data points indexed in time order
- Time introduces a unique set of problems

Note:
- Most of our data are time series
- Why is it difficult? Time importance, speed, shift, amplitude...
- How long, and how many data points in the past? Is it changing?

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## Time Series Tasks

- Forecasting
- Anomaly detection
- Clustering
- Classification

Note:
- What is the most important step in the analysis?

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## Data Preparation

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- Longest & the most important step
- Garbage in = garbage out!
- Try to gather some domain knowledge
- It is always application specific

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Note:
- Well-trained monkey can run fit & predict
- Be aware of what is your goal
- Scaling - mean, median, didn't I just lose some information?
- Arcsinh
- Many times way harder than other steps...

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### Feature Extraction

- Constructing combinations of the variables to get around time
- We can combine them with TS data


Note:
- Many times this is the best approach as we "know" the features
- Usually creating heterogeneous data
- Combination with TS is difficult
- Usage of NN to extract features (transformers, etc.)

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## Machine Learning Tasks

- Occam's razor
- Proper definition is key
- Start simple and advance gradually
- Try multiple ML families - there is no single best model

Note:
- People tend to blindly follow Internet guides
- heterogeneous vs homogeneous data

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## Anomaly Detection

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- What are anomalies?
- Time series anomalies
- Point outliers
- Sequence outliers
- Shape/behavior Anomaly

_{https://arxiv.org/pdf/2002.04236.pdf}
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Note:
- It's hard as there are not many anomalies
- Recall-precision trade-off
- Usually, noone can tell you if it's an anomaly
- Try to gather some "human" data

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### Anomaly Detection Methods

- STL decomposition
- Tree & density approaches (Isolation Forest, LODA)
- Neural Networks (Auto-Encoders)
- Forecasting


Note:
- STL - Robust, simple, well studied, Changes like Corona
- Tree and density - not really for time series
- Auto Encoders - NN problems (more on that later)
- Forecasting - error cumulation, is my forecast really good?

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### Isolation Forest

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- Binary tree
- Anomalies are "easy" to isolate
- Random orthogonal boundaries
- Scale invariance

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### LODA: Lightweight On-line Detector of Anomalies

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- Random projections and histograms ([1])
- Very fast and scalable
- Randomness adds robustness
- Anomaly explanation

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[1]: http://agents.fel.cvut.cz/stegodata/pdfs/Pev15-Loda.pdf

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## Forecasting

- Predicting the future from historic values
- Good error metric is essential!
- Can we predict a "random" series?


Note:
- Practical things
- Why do we want to forecast?
- Chaotic and random time series
- Do we have all the data
- Does our time series depend
- Good error metric is essential!
- We don't need a good prediction when nothing is happening

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### Model Validation

- Don't use the future to predict the future!
- Notice big changes in data (change points)
- Cross-validation & moving window


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### Forecasting Approaches

- STL decomposition
- Auto ARIMA
- Facebook Prophet
- Classic ML
- Time Frame Shifting
- Neural Networks
- Recurrent NN and LSTM
- CNN (SCINet)
- Decomposition (N-BEATS)
- Transformers (LTSF-Linear, Informer, etc.)

Note:
- Stability
- Cost vs effectiveness
- Always fact-check with a simple model
- heterogeneous vs homogeneous data
- Don't use a tank to kill a fly
- NN practical problems - Number of parameters (LSTM)

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### Facebook Prophet

- Decomposing TS as $y(t) = g(t) + s(t) + h(t) + et$


Note:
- Start with Auto-Arima
- Simple and automatic holiday support
- Kinda slow

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### N-BEATS


Note:
- Quite simple NN
- We can check output of every layer
- Still, we need to scale our data

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## Clasification & Clustering

- DTW, shape-based approaches, Hidden Markov Model
- Other ML approaches using DTW as metric or using DTW distance matrix


Note:
- 1NN DTW - surprisingly good and simple
- Time & Space complexity
- Not a metric nor derivable!
- sDTW - usage with NN
- HMM - complexity

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## Interesting Links

- Feature Extraction - [tsfresh](https://github.com/blue-yonder/tsfresh)
- Forecasting:
- [Sktime](https://github.com/sktime/sktime) & [tslearn](https://tslearn.readthedocs.io/en/stable/)
- Nixtla repositories
- [Stats Forecast](https://github.com/Nixtla/statsforecast)
- [Neural Forecast](https://github.com/Nixtla/neuralforecast)
- Auto Arima - [pmdarima](https://alkaline-ml.com/pmdarima/)

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Thank you for your attention!
Questions?