I am working on multi-class problem with sequences. My dataset is composed of sequences of data with different length.

E.g. 1500 labeled samples: 500 datapoint belongs to class A, 500 class B and 500 class C. For A and B the sequence length is 400 and 1000 respectively, and for class C the sequence length is 100.

In order to train the model I have applied post-padding on the sequences so that all the sequences have the same length. The resulting dataset has this shape (1500,1000).

I have tried first EMBEDDING+LSTM (mask_zero=True) to map and classify the sequences but even if the model achieve very high accuracy, evaluate the model with random/fake data it will classify based on the sequences' length: suggesting that the model is learning on the lengths instead of values.

The main problem is that the model is much more learning on 0s even if we use "mask_zero" into the embedding layer. My question is:

Does someone can suggest an approach to deal with very long sequences? Considering that we have very short and very long sequences to predict?

I am exploring another different approach:

  1. Train an Autoencoder (ANN or 1DCNN) to reduce the sequence length. Use the encoder and train again the Embedding layer + LSTM layer.


  • $\begingroup$ What kind of data is it? Text or numbers? Are the data points related to each other to try a compression method? $\endgroup$ Jul 26, 2021 at 18:06
  • $\begingroup$ Sequences are numbers. Each elements in a single sequence are related . what's the compression you suggests? $\endgroup$
    – scalessio
    Jul 29, 2021 at 10:33
  • $\begingroup$ If you use a logarithm, you can compress float data efficiently, but you might loss some information. For instance: log(20 000 000) = 7.03 and log(2 000) = 3.30. I used to apply this solution in similar cases than you have, and the results were good. $\endgroup$ Jul 29, 2021 at 13:41
  • $\begingroup$ how exactly should I apply this methodology? my dataset consists in a sequences of fixed length. if I understood I should apply the log to the length? $\endgroup$
    – scalessio
    Jul 29, 2021 at 15:02
  • $\begingroup$ Can you give an example of the values of a sequence please? $\endgroup$ Jul 29, 2021 at 15:17

1 Answer 1


Try the following:

  1. Check if text preprocessing is done correctly.
  2. With different sequence lengths (Ex: 1000, 750, 500 etc.)
  3. After the LSTM layer, try adding the ANN layers and check the results.
  4. Check the Transformers architecture for text classification. Reference: https://ai-brewery.medium.com/simple-chatbot-using-bert-and-pytorch-part-1-2735643e0baa
  • $\begingroup$ hi thanks for your reply. I don't need text processing since the sequences are already floating points values. And yes I have tried an architecture with embedding layer +lstm +ANN but it doesn't work: with random data the model predicts the classes based on sequence's length. $\endgroup$
    – scalessio
    Jul 29, 2021 at 10:35

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