docs for traditional metrics

docs/concepts/metrics/traditional.md

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+# Traditional NLP Metrics
+
+## Non LLM String Similarity
+
+he NonLLMStringSimilarity metric measures the similarity between the reference and the response using traditional string distance measures such as Levenshtein, Hamming, and Jaro. This metric is useful for evaluating the similarity of `response` to the `reference` text without relying on large language models (LLMs). The metric returns a score between 0 and 1, where 1 indicates a perfect match between the response and the reference. This is a non LLM based metric.
+
+### Example
+```python
+from ragas.dataset_schema import SingleTurnSample
+from ragas.metrics._string import NonLLMStringSimilarity
+
+sample = SingleTurnSample(
+    response="The Eiffel Tower is located in India.",
+    reference="The Eiffel Tower is located in Paris."
+)
+
+scorer = NonLLMStringSimilarity()
+await scorer.single_turn_ascore(sample)
+```
+
+One can choose from available string distance measures from `DistanceMeasure`. Here is an example of using Hamming distance.
+
+```python
+from ragas.metrics._string import NonLLMStringSimilarity, DistanceMeasure
+
+scorer = NonLLMStringSimilarity(distance_measure=DistanceMeasure.HAMMING)
+```
+
+
+## BLEU Score
+
+The [BLEU (Bilingual Evaluation Understudy)](https://en.wikipedia.org/wiki/BLEU) score is a metric used to evaluate the quality of `response` by comparing it with `reference`. It measures the similarity between the response and the reference based on n-gram precision and brevity penalty. BLEU score was originally designed to evaluate machine translation systems, but it is also used in other natural language processing tasks. Since it was designed to evaluate machine translation systems, it expects the response and reference to contain same number of sentences. The comparison is done at sentence level. BLEU score ranges from 0 to 1, where 1 indicates a perfect match between the response and the reference. This is a non LLM based metric.
+
+### Example
+```python
+from ragas.dataset_schema import SingleTurnSample
+from ragas.metrics._bleu_score import BleuScore
+
+sample = SingleTurnSample(
+    response="The Eiffel Tower is located in India.",
+    reference="The Eiffel Tower is located in Paris."
+)
+
+scorer = BleuScore()
+await scorer.single_turn_ascore(sample)
+```
+Custom weights may be supplied to fine-tune the BLEU score further. A tuple of float weights for unigrams, bigrams, trigrams and so on can be given by
+
+```python
+scorer = BleuScore(weights=(0.25, 0.25, 0.25, 0.25))
+```
+
+
+
+## ROUGE Score
+
+The [ROUGE (Recall-Oriented Understudy for Gisting Evaluation)](https://en.wikipedia.org/wiki/ROUGE_(metric)) score is a set of metrics used to evaluate the quality of natural language generations. It measures the overlap between the generated `response` and the `reference` text based on n-gram recall, precision, and F1 score. ROUGE score ranges from 0 to 1, where 1 indicates a perfect match between the response and the reference. This is a non LLM based metric.
+
+```python
+from ragas.dataset_schema import SingleTurnSample
+from ragas.metrics._rogue_score import RougeScore
+
+sample = SingleTurnSample(
+    response="The Eiffel Tower is located in India.",
+    reference="The Eiffel Tower is located in Paris."
+)
+
+scorer = RougeScore()
+await scorer.single_turn_ascore(sample)
+```
+
+You can change the `rouge_type` to `rouge-1`, `rouge-2`, or `rouge-l` to calculate the ROUGE score based on unigrams, bigrams, or longest common subsequence respectively.
+
+```python
+scorer = RougeScore(rouge_type="rouge-1")
+```
+
+You can change the `measure_type` to `precision`, `recall`, or `f1` to calculate the ROUGE score based on precision, recall, or F1 score respectively.
+
+```python
+scorer = RougeScore(measure_type="recall")
+```
+
+## Exact Match
+The ExactMatch metric checks if the response is exactly the same as the reference text. It is useful in scenarios where you need to ensure that the generated response matches the expected output word-for-word. For example, arguments in tool calls, etc. The metric returns 1 if the response is an exact match with the reference, and 0 otherwise.
+
+```python
+from ragas.dataset_schema import SingleTurnSample
+from ragas.metrics._string import ExactMatch
+
+sample = SingleTurnSample(
+    response="India",
+    reference="Paris"
+)
+
+scorer = ExactMatch()
+await scorer.single_turn_ascore(sample)
+```
+
+## String Presence
+The StringPresence metric checks if the response contains the reference text. It is useful in scenarios where you need to ensure that the generated response contains certain keywords or phrases. The metric returns 1 if the response contains the reference, and 0 otherwise.
+
+```python
+from ragas.dataset_schema import SingleTurnSample
+from ragas.metrics._string import StringPresence
+
+sample = SingleTurnSample(
+    response="The Eiffel Tower is located in India.",
+    reference="Eiffel Tower"
+)
+scorer = StringPresence()
+await scorer.single_turn_ascore(sample)
+```