# Semantic Textual Similarity

Semantic Textual Similarity (STS) assigns a score on the similarity of two texts. In this example, we use the [STSbenchmark](https://ixa2.si.ehu.es/stswiki/index.php/STSbenchmark) as training data to fine-tune our network. See the following example scripts how to tune SentenceTransformer on STS data:

- **[training_stsbenchmark.py](training_stsbenchmark.py)** - This example shows how to create a SentenceTransformer model from scratch by using a pre-trained transformer model together with a pooling layer.
 - **[training_stsbenchmark_continue_training.py](training_stsbenchmark_continue_training.py)** - This example shows how to continue training on STS data for a previously created & trained SentenceTransformer model. In that example, we load a model trained on [NLI data](../nli/README.md).
 

## Training data
In STS, we have sentence pairs annotated together with a score indicating the similarity. For the [STSbenchmark](https://ixa2.si.ehu.es/stswiki/index.php/STSbenchmark), the scores ranges from 0 (the content of the two sentences are competely different) up to 5 (the two sentences are identical in terms of their meaning). To train our network, we need to normalize these scores to a range of 0-1. This can simply be done by dividing the score by 5.

To store our training data, we create a list with `InputExample` objects. Each `InputExample` contains the sentence pair together with the label (score) that ranges between 0 - 1. A simplified version how the training data has to look like is the following:

```python
from sentence_transformers import SentenceTransformer, SentencesDataset, InputExample, losses

model = SentenceTransformer('nli-distilroberta-base-v2')
train_examples = [InputExample(texts=['My first sentence', 'My second sentence'], label=0.8),
    InputExample(texts=['Another pair', 'Unrelated sentence'], label=0.3)]
train_dataset = SentencesDataset(train_examples, model)
```

## Loss Function
As loss function we use [CosineSimilarityLoss](../../../docs/package_reference/losses.html#cosinesimilarityloss).


*CosineSimilarityLoss* trains the network with a siamese network structure (for details see: [Sentence-BERT: Sentence Embeddings using Siamese BERT-Networks](https://arxiv.org/abs/1908.10084))


![SBERT Siamese Network Architecture](https://raw.githubusercontent.com/UKPLab/sentence-transformers/master/docs/img/SBERT_Siamese_Network.png "SBERT Siamese Architecture")


For each sentence pair, we pass sentence A and sentence B through our network which yields the embeddings *u* und *v*. The similarity of these embeddings is computed using cosine similarity and the result is compared to the gold similarity score. This allows our network to be fine-tuned and to recognize the similarity of sentences. 

This training in a siamese network structure is done automatically when we use CosineSimilarityLoss.