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I am doing a supervised binary text classification task.

I want to classify the texts from site A, site B, and site C.

The in-domain performance looks OK for texts of each site. (92%-94% accuracy).

However, if I applied the model trained on texts of one site directly onto texts of another site(without fine-tuning), the performance downgrades a lot. (7%-16% downgrade for accuracy).

Approaches I already tried:

  1. Doc2vec embedding(trained on texts from one site) + logistic regression.

  2. Bert embedding + logistic regression. (Using bert-as-a-service to generate the embeddings based on google pre-trained bert models).

  3. TF-IDF + logistic regression.

  4. Pre-trained Word2vec embedding(average word embedding for text) + logistic regression.

All of those approaches don't work very well.

I knew that the performance downgrade is unavoidable, but I would like to get a maybe 3% - 5% downgrade.

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  • $\begingroup$ I don't understand: are you training 3 different binary models or a single 3-way classification model? and if you're doing 3 different binary models, what are the negative instances used for training? $\endgroup$ – Erwan Jul 22 '20 at 1:11
  • $\begingroup$ @Erwan, I am training 3 different binary models on texts from three different sites separately. When I applied the binary model trained on one site onto texts of another site, the performance downgrades a lot. $\endgroup$ – LGDGODV Jul 22 '20 at 4:37
  • $\begingroup$ Ok, so I assume that you have negative instances in your training data for every site right? Where do you take these instances from? Btw it looks like what you need is one class classification, because with text it's impossible to have a fully representative sample of "not from this site". There's also the question of whether these sites should be distinguished based on semantic domain/topic or genre/writing style, currently your methods are more suitable for the former (which might be what you want). $\endgroup$ – Erwan Jul 22 '20 at 11:07
  • $\begingroup$ @Erwan, thanks for your reply. Yes, data for each site contains the same number of positive instances(related to a topic I want to classify) and negative instances(randomly selected among all other topics of the site). I just wondering whether I can apply a pre-trained classifier on one site directly onto another site to classify those positive instances. $\endgroup$ – LGDGODV Jul 22 '20 at 19:07
  • $\begingroup$ @Erwan Domains of those sites are similar, but the focus of the text could be different among sites, and those sites clearly have different genre/writing style. I want to build a classifier that could classify texts based on the semantic domain/topic and are't affected by genre/writing style. I will try one class classification. $\endgroup$ – LGDGODV Jul 22 '20 at 19:09
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Generally the task of recognizing one type of text against "anything else" is a quite difficult problem, since there is so much diversity in text that there cannot be any good representative sample of "anything else".

Typically this problem is treated as a one-class classification problem: the idea is for the learning algorithm to capture what represents the positive class only, considering anything else as negative. To my knowledge this is used mostly for author identification and related stylometry tasks. The PAN workshop series offer a good deal of state of the arts methods and datasets around these tasks.

It is also possible to frame the problem as binary classification, but then one must be very creative with the negative instances in the training set. Probably the main problem with your current approach is this: your negative instances are only "randomly selected among all other topics of the site". This means that the classifier knows only texts from the site on which it is trained, so it has no idea what to do with any new text which doesn't look like anything seen in the training data. A method which has been used to increase the diversity of the negative instances is to automatically generate google queries with a few random words which appear in one of the positive instances, then download whatever text Google retrieves as negative instance.

Another issue with binary classification is the distribution of positive/negative instances: if you train a model with 50/50 positive/negative, the model expects that by default there is 50% chance for each. This can cause a huge bias when applied to a test set which contains mostly negative instances, especially if these don't look like the negative instances seen during training.

Finally be careful about the distinction semantic topic vs. writing style, because the features for these two are usually very different: in the former case the stop words are usually removed, the content words (nouns, verbs, adjectives) are important (hence one uses things like TFIDF). In the latter it's the opposite: stop words and punctuation should be kept (because they are good indicators of writing style) whereas content words are removed because they tend to bias the model the topic instead of the style. In stylometry features based on characters n-grams have been shown to perform well... even though it's not very clear why it works!

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