Overview: Evaluating NLP Models

  • Natural Language Processing has revolutionized the way we interact with machines and interpret textual data. Models in NLP are designed to understand, interpret, and generate human-like text, making them integral in fields like text analysis, translation, summarization, and information extraction.
  • Evaluating the performance of these models is crucial for their development and improvement. In this regard, several metrics have been proposed, such as BLEU, ROUGE, and the F1 score, each of which serves different purposes and has its own strengths and weaknesses.

BLEU (Bilingual Evaluation Understudy)

  • BLEU, an acronym for Bilingual Evaluation Understudy, is predominantly used in machine translation. It quantifies the quality of the machine-generated text by comparing it with a set of reference translations. The crux of the BLEU score calculation is the precision of n-grams (continuous sequence of n items in text) in the machine-translated text. However, to prevent the overestimation of precision due to shorter sentences, BLEU includes a brevity penalty factor. Despite its widespread use, it’s important to note that BLEU mainly focuses on precision, and lacks a recall component.
  • Mathematically, precision for unigram (single word) is calculated as follows:
  • Precision = (Number of correct words in machine translation) / (Total words in machine translation)
  • BLEU extends this idea to consider precision of n-grams (continuous sequence of n items from a given sample of text). However, BLEU uses a modified precision calculation to avoid the problem of artificially inflated precision scores.
  • The equation of BLEU score for n-grams is:

  • \[BLEU = BP * exp ( Sum (from i=1 to n) w_i * log (p_i) )\]


  • BP is the brevity penalty (to penalize short sentences)
  • w_i are the weights for each gram (usually, we give equal weight)
  • p_i is the precision for each i-gram
  • Use when: Evaluating Machine Translation models.
  • Why: BLEU is effective in assessing the closeness of machine-generated translations to a set of high-quality reference translations. It’s suitable when precision of translated text is a priority.
  • Limitation: It may not capture the fluency or grammatical correctness of the translation, as it primarily focuses on the precision of n-grams.

    A metric for evaluating a generated text’s quality by comparing it to reference texts, focusing on the precision of word sequences.

ROUGE (Recall-Oriented Understudy for Gisting Evaluation)

  • ROUGE, which stands for Recall-Oriented Understudy for Gisting Evaluation, is used primarily for evaluating automatic summarization and, sometimes, machine translation. The key feature of ROUGE is its focus on recall, measuring how many of the reference n-grams are found in the system-generated summary. This makes it especially useful for tasks where coverage of key points is important. Among its variants, ROUGE-N computes the overlap of n-grams, ROUGE-L uses the longest common subsequence to account for sentence-level structure similarity, and ROUGE-S includes skip-bigram statistics.
  • ROUGE-N specifically refers to the overlap of N-grams between the system and reference summaries.

  • \[ROUGE-N = (Number of N-grams in both system and reference summary) / (Total number of N-grams in reference summary)\]
  • ROUGE-L considers sentence level structure similarity naturally and identifies longest co-occurring in-sequence n-grams automatically.
  • ROUGE-S includes skip-bigram plus unigram-based co-occurrence statistics. Skip-bigram is any pair of words in their sentence order.
  • Use when: Evaluating Automatic Summarization and Machine Translation (to a lesser extent).
  • Why: ROUGE is useful when the coverage of the reference content is important, especially in summarization tasks. It measures how many of the reference n-grams are captured in the generated summary.
  • Variants: Choose ROUGE-N for evaluating n-gram overlap, ROUGE-L for sentence-level structure similarity, and ROUGE-S for skip-bigram based evaluation.

    Used mainly in summarization, this metric assesses the quality of a summary by measuring its overlap with reference summaries, emphasizing recall.

F1 Score

  • The F1 score, commonly used in many machine learning classification problems, is also applicable to various NLP tasks like Named Entity Recognition, POS-tagging, etc. The F1 score is the harmonic mean of precision and recall, and thus balances the two and prevents extreme cases where one is favored over the other. It ranges from 0 to 1, where 1 signifies perfect precision and recall.
  • Precision is the number of true positive results divided by the number of all positive results, including those not identified correctly. Recall, on the other hand, is the number of true positive results divided by the number of all samples that should have been identified as positive.
  • The F1 score is the harmonic mean of the precision and recall, where an F1 score reaches its best value at 1 (perfect precision and recall) and worst at 0.
  • The mathematical representation is as follows:
  • \[F1 Score = 2 * (Precision * Recall) / (Precision + Recall)\]
  • In NLP, F1 score is often used in tasks like Named Entity Recognition, POS-tagging, etc.
  • Use when: Evaluating tasks like Named Entity Recognition (NER), Part-of-Speech (POS) tagging, and other classification tasks within NLP.
  • Why: The F1 score is ideal when you need to balance precision and recall, especially in cases where both false positives and false negatives are equally costly.

    A balanced metric combining precision and recall, often used in classification tasks to measure a model’s accuracy.


  • What is Perplexity?
    • Perplexity is a measure commonly used in natural language processing and information theory to assess how well a probability distribution predicts a sample. In the context of language models, it evaluates the uncertainty of a model in predicting the next word in a sequence.
  • Why use Perplexity?
    • Perplexity serves as an inverse probability metric. A lower perplexity indicates that the model’s predictions are closer to the actual outcomes, meaning the model is more confident (and usually more accurate) in its predictions.
  • How is it calculated?
    • For a probability distribution \(p\) and a sequence of \(N\) words \(w_1, w_2, ... w_N\):
\[\text{Perplexity} = p(w_1, w_2, ... w_N)^{-\frac{1}{N}}\]
  • In simpler terms, if we only consider bigrams (two-word sequences) and a model assigns a probability \(p\) to the correct next word, the perplexity would be \(\frac{1}{p}\).

  • Where to use?
    1. Language Models: To evaluate the quality of language models. A model with lower perplexity is generally considered better.
    2. Model Comparison: To compare different models or different versions of the same model over a dataset.
  • Use when: Evaluating Language Models, such as those used in text generation.
  • Why: Perplexity measures how well a probability model predicts a sample. A lower perplexity score indicates a model is better at predicting a sequence of words.

    A measure of how well a probability model predicts a sample, often used in language modeling to evaluate the model’s ability to predict the next word. Higher Perplexity = Human written text: Generally suggests that the text is less predictable or more complex. In the context of language models, a higher perplexity might indicate that the model is less certain about its predictions or that the text has a more complex structure or vocabulary. Lower Perplexity = AI generated text: Indicates that the text is more predictable or simpler. For language models, a lower perplexity usually means the model is more confident in its predictions and the text may follow more common linguistic patterns.


  • What is Burstiness?
    • Burstiness refers to the occurrence of unusually frequent repetitions of certain terms in a text. It’s the idea that once a word appears, it’s likely to appear again in short succession.
  • Why consider Burstiness?
    • Burstiness can indicate certain patterns or biases in text generation. For instance, if an AI language model tends to repeat certain words or phrases too often in its output, it may suggest an over-reliance on certain patterns or a lack of diverse responses.
  • How is it measured?
    • While there isn’t a single standard way to measure burstiness, one common method involves looking at the distribution of terms and identifying terms that appear more frequently than a typical random distribution would suggest.
  • Where to use?
    1. Text Analysis: To understand patterns in text, e.g., to see if certain terms are being repeated unusually often.
    2. Evaluating Generative Models: If a language model produces text with high burstiness, it might be overfitting to certain patterns in the training data or lacking diversity in its outputs.
  • In Context of AI and Recommender Systems:
    • Both metrics can provide insights into the behavior of AI models, especially generative ones like LLMs.
  • Perplexity can tell us how well the model predicts or understands a given dataset.
  • Burstiness can inform us about the diversity and variability of the model’s outputs.

  • In recommender systems, if the system is generating textual recommendations or descriptions, perplexity can help assess the quality of those recommendations. Burstiness might indicate if the system keeps recommending the same/similar items repetitively.
  • Use when: Analyzing text generation models for diversity and pattern detection.
  • Why: Burstiness helps identify if a language model is over-reliant on certain words or phrases, indicating a lack of diversity or potential overfitting in its outputs.

    Evaluates the variation in sentence length in a text, indicating the naturalness and dynamism of generated language. Higher Burstiness = Human written text: Means there’s a greater variation in sentence length within the text. It could suggest a more dynamic or natural writing style, as human writing often varies in sentence length. Lower Burstiness: Indicates more uniform sentence lengths, which might suggest a more monotonous or mechanical style.

METEOR (Metric for Evaluation of Translation with Explicit ORdering)

  • METEOR, which stands for Metric for Evaluation of Translation with Explicit ORdering, is another metric used for evaluating machine translation. Unlike BLEU, METEOR emphasizes both precision and recall, taking into account the number of matching words between the machine-generated text and reference translations. It’s known for using synonyms and stemming to match words, allowing for a more flexible comparison.
  • METEOR calculates a score based on the harmonic mean of precision and recall, giving equal importance to both. It also includes a penalty for too many unmatched words, ensuring that translations are not just accurate but also coherent and fluent.
  • The formula for METEOR is as follows:
\[\text{METEOR} = \frac{10 \cdot P \cdot R}{R + 9 \cdot P} - \text{Penalty}\]


  • P is the precision (proportion of matched words in the machine translation)
  • R is the recall (proportion of matched words in the reference translation)
  • Penalty is applied for word order differences.
  • Use when: Evaluating Machine Translation, especially when both accuracy and fluency are important.
  • Why: METEOR is more nuanced than BLEU as it considers synonyms and stemming, and balances precision with recall, thus better evaluating the overall quality of translations.

    An advanced metric for evaluating translation quality, considering exact word matches, synonymy, and word order.


  • BERTScore is a more recent metric for evaluating the quality of text. It leverages the contextual embeddings from BERT (Bidirectional Encoder Representations from Transformers), a powerful language model. This allows for a more nuanced comparison of text, as BERTScore can understand the context in which words are used.
  • BERTScore computes the cosine similarity between the embeddings of words in the candidate text and the reference text, accounting for the deep semantic similarity rather than just surface-level overlap.
  • The calculation involves finding the best match for each word in the candidate text within the reference text and averaging these scores.
  • BERTScore is especially useful in tasks where understanding the context and the meaning of words in that context is crucial, such as in dialogue systems and more complex translation tasks.
  • Use when: Evaluating tasks where deep semantic understanding is crucial, like machine translation, text summarization, and dialogue systems.
  • Why: BERTScore leverages contextual embeddings, offering a sophisticated method to assess semantic similarity between generated and reference texts.

    Utilizes the contextual embeddings from BERT models to evaluate the semantic similarity between generated and reference texts.


  • MoverScore is a metric that also utilizes contextual word embeddings, like those from BERT or other transformer-based models, for evaluating text generation tasks.
  • The key concept behind MoverScore is the use of Earth Mover’s Distance (EMD) to measure the distance between the distributions of contextual word embeddings in the generated and reference texts.
  • MoverScore effectively captures semantic meaning and is particularly useful in scenarios where lexical overlap metrics like BLEU or ROUGE might miss nuances in meaning.
  • This metric is valuable in evaluating tasks like text summarization, translation, and any other task where semantic understanding is critical.
  • Use when: Evaluating tasks that require a nuanced understanding of semantic content, similar to BERTScore.
  • Why: MoverScore uses contextual embeddings and Earth Mover’s Distance to assess semantic similarity, making it suitable for tasks where traditional lexical overlap metrics fall short.

    A metric that compares semantic representations of text at the sentence level, leveraging contextual word embeddings for more nuanced evaluation.

Use case

RAG model in a Q&A context

    • Use when: You are focusing on the linguistic quality of the generated answers in comparison to a reference answer.
    • Why: These metrics are useful for evaluating the closeness of the generated text to a standard reference, which is important in Q&A to ensure that the answers are not only accurate but also appropriately phrased.
    • Use when: The completeness of the answer is more important than its exact wording.
    • Why: ROUGE is effective for evaluating how much of the key information from the reference texts is captured in the generated answers, which is crucial for Q&A systems to ensure they are providing complete information.
  • F1 Score
    • Use when: Evaluating the model on specific-answer tasks, like factoid Q&A, where answers are either right or wrong.
    • Why: The F1 score can effectively balance precision and recall, making it suitable for tasks where you need to identify correct answers amidst potentially many generated responses.
  • BERTScore or MoverScore
    • Use when: Deep semantic understanding and context matching of the answers are critical.
    • Why: These metrics leverage advanced language model embeddings and are capable of capturing semantic similarities between the generated answer and the reference, which is crucial for complex Q&A tasks.