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| title: Perplexity | |
| emoji: 🤗 | |
| colorFrom: blue | |
| colorTo: red | |
| sdk: gradio | |
| sdk_version: 3.19.1 | |
| app_file: app.py | |
| pinned: false | |
| tags: | |
| - evaluate | |
| - metric | |
| description: >- | |
| Perplexity (PPL) is one of the most common metrics for evaluating language models. | |
| It is defined as the exponentiated average negative log-likelihood of a sequence, calculated with exponent base `e`. | |
| For more information on perplexity, see [this tutorial](https://huggingface.co/docs/transformers/perplexity). | |
| # Metric Card for Perplexity | |
| ## Metric Description | |
| Given a model and an input text sequence, perplexity measures how likely the model is to generate the input text sequence. | |
| As a metric, it can be used to evaluate how well the model has learned the distribution of the text it was trained on. | |
| In this case, `model_id` should be the trained model to be evaluated, and the input texts should be the text that the model was trained on. | |
| This implementation of perplexity is calculated with log base `e`, as in `perplexity = e**(sum(losses) / num_tokenized_tokens)`, following recent convention in deep learning frameworks. | |
| ## Intended Uses | |
| Any language generation task. | |
| ## How to Use | |
| The metric takes a list of text as input, as well as the name of the model used to compute the metric: | |
| ```python | |
| from evaluate import load | |
| perplexity = load("perplexity", module_type="metric") | |
| results = perplexity.compute(predictions=predictions, model_id='gpt2') | |
| ``` | |
| ### Inputs | |
| - **model_id** (str): model used for calculating Perplexity. NOTE: Perplexity can only be calculated for causal language models. | |
| - This includes models such as gpt2, causal variations of bert, causal versions of t5, and more (the full list can be found in the AutoModelForCausalLM documentation here: https://huggingface.co/docs/transformers/master/en/model_doc/auto#transformers.AutoModelForCausalLM ) | |
| - **predictions** (list of str): input text, where each separate text snippet is one list entry. | |
| - **batch_size** (int): the batch size to run texts through the model. Defaults to 16. | |
| - **add_start_token** (bool): whether to add the start token to the texts, so the perplexity can include the probability of the first word. Defaults to True. | |
| - **device** (str): device to run on, defaults to `cuda` when available | |
| ### Output Values | |
| This metric outputs a dictionary with the perplexity scores for the text input in the list, and the average perplexity. | |
| If one of the input texts is longer than the max input length of the model, then it is truncated to the max length for the perplexity computation. | |
| ``` | |
| {'perplexities': [8.182524681091309, 33.42122268676758, 27.012239456176758], 'mean_perplexity': 22.871995608011883} | |
| ``` | |
| The range of this metric is [0, inf). A lower score is better. | |
| #### Values from Popular Papers | |
| ### Examples | |
| Calculating perplexity on predictions defined here: | |
| ```python | |
| perplexity = evaluate.load("perplexity", module_type="metric") | |
| input_texts = ["lorem ipsum", "Happy Birthday!", "Bienvenue"] | |
| results = perplexity.compute(model_id='gpt2', | |
| add_start_token=False, | |
| predictions=input_texts) | |
| print(list(results.keys())) | |
| >>>['perplexities', 'mean_perplexity'] | |
| print(round(results["mean_perplexity"], 2)) | |
| >>>646.75 | |
| print(round(results["perplexities"][0], 2)) | |
| >>>32.25 | |
| ``` | |
| Calculating perplexity on predictions loaded in from a dataset: | |
| ```python | |
| perplexity = evaluate.load("perplexity", module_type="metric") | |
| input_texts = datasets.load_dataset("wikitext", | |
| "wikitext-2-raw-v1", | |
| split="test")["text"][:50] | |
| input_texts = [s for s in input_texts if s!=''] | |
| results = perplexity.compute(model_id='gpt2', | |
| predictions=input_texts) | |
| print(list(results.keys())) | |
| >>>['perplexities', 'mean_perplexity'] | |
| print(round(results["mean_perplexity"], 2)) | |
| >>>576.76 | |
| print(round(results["perplexities"][0], 2)) | |
| >>>889.28 | |
| ``` | |
| ## Limitations and Bias | |
| Note that the output value is based heavily on what text the model was trained on. This means that perplexity scores are not comparable between models or datasets. | |
| See Meister and Cotterell, ["Language Model Evaluation Beyond Perplexity"]( https://arxiv.org/abs/2106.00085) (2021) for more information about alternative model evaluation strategies. | |
| ## Citation | |
| ```bibtex | |
| @article{jelinek1977perplexity, | |
| title={Perplexity—a measure of the difficulty of speech recognition tasks}, | |
| author={Jelinek, Fred and Mercer, Robert L and Bahl, Lalit R and Baker, James K}, | |
| journal={The Journal of the Acoustical Society of America}, | |
| volume={62}, | |
| number={S1}, | |
| pages={S63--S63}, | |
| year={1977}, | |
| publisher={Acoustical Society of America} | |
| } | |
| ``` | |
| ## Further References | |
| - [Hugging Face Perplexity Blog Post](https://huggingface.co/docs/transformers/perplexity) | |