An all-MLP architecture for time series forecasting – Google Research Blog

Replace — 2023/09/12: This put up has been up to date because the work described is now revealed in Transactions on Machine Studying Analysis.

Time sequence forecasting is vital to numerous real-world purposes, from demand forecasting to pandemic unfold prediction. In multivariate time sequence forecasting (forecasting a number of variants on the identical time), one can break up current strategies into two classes: univariate fashions and multivariate fashions. Univariate fashions deal with inter-series interactions or temporal patterns that embody developments and seasonal patterns on a time sequence with a single variable. Examples of such developments and seasonal patterns is perhaps the best way mortgage charges improve as a consequence of inflation, and the way site visitors peaks throughout rush hour. Along with inter-series patterns, multivariate fashions course of intra-series options, often called cross-variate info, which is very helpful when one sequence is a sophisticated indicator of one other sequence. For instance, an increase in physique weight could trigger a rise in blood strain, and growing the value of a product could result in a lower in gross sales. Multivariate fashions have just lately develop into standard options for multivariate forecasting as practitioners consider their functionality of dealing with cross-variate info could result in higher efficiency.

In recent times, deep studying Transformer-based architectures have develop into a preferred selection for multivariate forecasting fashions as a consequence of their superior efficiency on sequence duties. Nonetheless, superior multivariate fashions carry out surprisingly worse than easy univariate linear fashions on commonly-used long-term forecasting benchmarks, corresponding to Electrical energy Transformer Temperature (ETT), Electrical energy, Site visitors, and Climate. These outcomes elevate two questions:

Does cross-variate info profit time sequence forecasting?

When cross-variate info shouldn’t be useful, can multivariate fashions nonetheless carry out in addition to univariate fashions?

In “TSMixer: An All-MLP Structure for Time Collection Forecasting”, revealed within the Transactions on Machine Studying Analysis (TMLR), we analyze some great benefits of univariate linear fashions and reveal their effectiveness. Insights from this evaluation lead us to develop Time-Collection Mixer (TSMixer), a sophisticated multivariate mannequin that leverages linear mannequin traits and performs effectively on long-term forecasting benchmarks. To the perfect of our information, TSMixer is the primary multivariate mannequin that performs in addition to state-of-the-art univariate fashions on long-term forecasting benchmarks, the place we present that cross-variate info is much less useful. To display the significance of cross-variate info, we consider a tougher real-world utility, M5. Lastly, empirical outcomes present that TSMixer outperforms state-of-the-art fashions, corresponding to PatchTST, Fedformer, Autoformer, DeepAR and TFT.

TSMixer structure

A key distinction between linear fashions and Transformers is how they seize temporal patterns. On one hand, linear fashions apply mounted and time-step-dependent weights to seize static temporal patterns, and are unable to course of cross-variate info. Then again, Transformers use consideration mechanisms that apply dynamic and data-dependent weights at every time step, capturing dynamic temporal patterns and enabling them to course of cross-variate info.

In our evaluation, we present that underneath frequent assumptions of temporal patterns, linear fashions have naïve options to completely get better the time sequence or place bounds on the error, which implies they’re nice options for studying static temporal patterns of univariate time sequence extra successfully. In distinction, it’s non-trivial to search out related options for consideration mechanisms, because the weights utilized to every time step are dynamic. Consequently, we develop a brand new structure by changing Transformer consideration layers with linear layers. The ensuing TSMixer mannequin, which is analogous to the pc imaginative and prescient MLP-Mixer technique, alternates between purposes of the multi-layer perceptron in several instructions, which we name time-mixing and feature-mixing, respectively. The TSMixer structure effectively captures each temporal patterns and cross-variate info, as proven within the determine beneath. The residual designs make sure that TSMixer retains the capability of temporal linear fashions whereas nonetheless with the ability to exploit cross-variate info.

Transformer block and TSMixer block architectures. TSMixer replaces the multi-head consideration layer with time-mixing, a linear mannequin utilized on the time dimension.

Comparability between data-dependent (consideration mechanisms) and time-step-dependent (linear fashions). That is an instance of forecasting the subsequent time step by studying the weights of the earlier three time steps.

Analysis on long-term forecasting benchmarks

We consider TSMixer utilizing seven standard long-term forecasting datasets (ETTm1, ETTm2, ETTh1, ETTh2, Electrical energy, Site visitors, and Climate), the place latest analysis has proven that univariate linear fashions outperform superior multivariate fashions with giant margins. We examine TSMixer with state-of-the-art multivariate fashions (TFT, FEDformer, Autoformer, Informer), and univariate fashions, together with linear fashions and PatchTST. The determine beneath reveals the common enchancment of imply squared error (MSE) by TSMixer in contrast with others. The common is calculated throughout datasets and a number of forecasting horizons. We display that TSMixer considerably outperforms different multivariate fashions and performs on par with state-of-the-art univariate fashions. These outcomes present that multivariate fashions are able to performing in addition to univariate fashions.

The common MSE enchancment of TSMixer in contrast with different baselines. The crimson bars present multivariate strategies and the blue bars present univariate strategies. TSMixer achieves important enchancment over different multivariate fashions and achieves comparable outcomes to univariate fashions.

Ablation examine

We carried out an ablation examine to match TSMixer with TMix-Solely, a TSMixer variant that consists of time mixing layers solely. The outcomes present that TMix-Solely performs nearly the identical as TSMixer, which implies the extra function mixing layers don’t enhance the efficiency and confirms that cross-variate info is much less useful on standard benchmarks. The outcomes validate the superior univariate mannequin efficiency proven in earlier analysis. Nonetheless, current long-term forecasting benchmarks are usually not effectively consultant of the necessity for cross-variate info in some real-world purposes the place time sequence could also be intermittent or sparse, therefore temporal patterns will not be ample for forecasting. Due to this fact, it might be inappropriate to judge multivariate forecasting fashions solely on these benchmarks.

Analysis on M5: Effectiveness of cross-variate info

To additional display the good thing about multivariate fashions, we consider TSMixer on the difficult M5 benchmark, a large-scale retail dataset containing essential cross-variate interactions. M5 comprises the knowledge of 30,490 merchandise collected over 5 years. Every product description contains time sequence information, like every day gross sales, promote value, promotional occasion info, and static (non-time-series) options, corresponding to retailer location and product class. The purpose is to forecast the every day gross sales of every product for the subsequent 28 days, evaluated utilizing the weighted root imply sq. scaled error (WRMSSE) from the M5 competitors. The sophisticated nature of retail makes it tougher to forecast solely utilizing univariate fashions that concentrate on temporal patterns, so multivariate fashions with cross-variate info and even auxiliary options are extra important.

First, we examine TSMixer to different strategies solely contemplating the historic information, corresponding to every day gross sales and historic promote costs. The outcomes present that multivariate fashions outperforms univariate fashions considerably, indicating the usefulness of cross-variate info. And amongst all in contrast strategies, TSMixer successfully leverages the cross-variate info and achieves the perfect efficiency.

Moreover, to leverage extra info, corresponding to static options (e.g., retailer location, product class) and future time sequence (e.g., a promotional occasion scheduled in coming days) supplied in M5, we suggest a precept design to increase TSMixer. The prolonged TSMixer aligns various kinds of options into the identical size, after which applies a number of mixing layers to the concatenated options to make predictions. The prolonged TSMixer structure outperforms fashions standard in industrial purposes, together with DeepAR and TFT, showcasing its robust potential for real-world impression.

The structure of the prolonged TSMixer. Within the first stage (align stage), it aligns the various kinds of options into the identical size earlier than concatenating them. Within the second stage (mixing stage) it applies a number of mixing layers conditioned with static options.

The WRMSSE on M5. The primary three strategies (blue) are univariate fashions. The center three strategies (orange) are multivariate fashions that think about solely historic options. The final three strategies (crimson) are multivariate fashions that think about historic, future, and static options.

Conclusion

We current TSMixer, a sophisticated multivariate mannequin that leverages linear mannequin traits and performs in addition to state-of-the-art univariate fashions on long-term forecasting benchmarks. TSMixer creates new potentialities for the event of time sequence forecasting architectures by offering insights into the significance of cross-variate and auxiliary info in real-world situations. The empirical outcomes spotlight the necessity to think about extra sensible benchmarks for multivariate forecasting fashions in future analysis. We hope that this work will encourage additional exploration within the area of time sequence forecasting, and result in the event of extra highly effective and efficient fashions that may be utilized to real-world purposes.

Acknowledgements

This analysis was performed by Si-An Chen, Chun-Liang Li, Nate Yoder, Sercan O. Arik, and Tomas Pfister.