[FEATURE] Scaler with rolling/expanding window to eliminate look ahead bias

[tuomijal]

Problem description
Currently, Scaler transforms input series "globally", meaning that all values of the input vector are considered:

from darts.dataprocessing.transformers import Scaler
from darts import TimeSeries

scaler = Scaler()

s = [1, 2, 3, 4, 5, 6, 7, 8, 9]
s = TimeSeries.from_series(s)

s_norm = scaler.fit_transform(s)
s_norm.pd_series()

0    0.000
1    0.125
2    0.250
3    0.375
4    0.500
5    0.625
6    0.750
7    0.875
8    1.000

This is not a problem if data is manually split into train and test sets and scaler is fitted with training set.

However, if we go on to use this vector as an input to historical forecasts, we risk introducing look ahead bias into analysis (at least when performing normalization). To eliminate this bias, rolling window approach is sometimes used https://arxiv.org/abs/1907.09452.

Describe proposed solution
One solution would be to add parameters to scaler like so:

class Scaler(InvertibleDataTransformer, FittableDataTransformer):
    def __init__(
        self, scaler=None, type="Global", window=None, name="Scaler", n_jobs: int = 1, verbose: bool = False
    ):
   """
   Parameters
   ----------
   type
        The type to scale the data. Options:
        "Global" uses all data points 
        "Rolling" uses rolling window with window size specified by 'window' parameter
        "Expanding" uses expanding window with initial window size specified by 'window' parameter
    window
        Size of window if type is either "Rolling" or "Expanding"
    """

Describe potential alternatives
Another option is to integrate this functionality to historical_forecasts and backtest functions. This might be convenient because parameters above could be inferred by the desired backtesting setup.

Additional context
Thank you again for excellent software!

[dennisbader]

Another way would be to allow users to pass some (of our existing) transformers for the target and covariates to historical_forecasts.
Then we could simply refit transform on each train eval split.

[hrzn]

Thanks for raising this excellent point @tuomijal. I personally like the solution of @dennisbader as it would remove the need for users to use the windowing exactly right - they wouldn't have to worry about it, only specify which kind of scaling they want when calling historical forecasts.

[tuomijal]

I agree, the solution proposed by @dennisbader is the most elegant one 👍🏼

[JanFidor]

Hi @dennisbader @madtoinou ! The PR looks cool, could I pick it up?

[madtoinou]

Hi @JanFidor,

historical_forecast() currently contains several bugs that are being fixed, and will undergo a considerable refactoring after the next release. I would recommend waiting for these changes to be merged before working on this very interesting feature.

[JanFidor]

Sure things, If there's a chance to avoid major merge conflicts, I'll happily take it. I'll keep my eyes peeled for the new release!

[madtoinou]

@JanFidor

Refactoring of historical forecasts has just been merged on the main branch, if you still have time to work on this, you can go ahead!

The logic is now found in two different places: ForecastingModel.historical_forecasts and RegressionModel._optimized_historical_forecasts, make sure to implement this in both. If you need, you can implement this feature in utils/historical_forecasts/utils.py and call it in the two methods.

[JanFidor]

Hi @madtoinou, thanks for reminding me, this issue totally slipped my mind! Small heads up, I might have slightly less time going forward, but I'll happily give it a go. I already browsed the RegressionModel._optimized_historical_forecasts method and noticed that it's only used with pretrained models (I think that in this case data leakage would have to be prevented outside of historical_forecasts.), so I wanted to ask you whether I'm missing something or if I should just add a Scaler as an unused parameter for now.

[madtoinou]

Indeed, we decided to optimize this method step by step and the "retrain" logic was a bit harder to support directly.

I think that the main source of data leakage is the processing/transformation of the entire input series instead of just the part available/used for the latest historical forecast (for both retraining and/or inference). So the logic should be contained in historical_forecasts().

[j-adamczyk]

Any news on this? Lack of this feature means that if backcast or other more involved testing procedures are needed, Darts is quite unusable, since those transforms are really necessary (e.g. differencing, scaling).

[Joseph-Foley]

Yeah, I'm also quite keen on this being part of darts. I was hoping the pipeline class would function more like sklearns pipeline where transformations and models can be bundled together. Then if the pipeline class had backtest / historical_forecast we could be sure of no data leakage during backtesting.

[joepvdbogaert]

+1 this is crucial to be able to user darts' backtest functionality