---
layout: default
title: Ranking evaluation
parent: REST API reference
nav_order: 82
---

# Ranking evaluation

The [rank]({{site.url}}{{site.baseurl}}/opensearch/supported-field-types/rank/) eval endpoint allows you to evaluate the quality of ranked search results.

## Path and HTTP methods

```
GET <index_name>/_rank_eval 
POST <index_name>/_rank_eval
```

## Query parameters

Query parameters are optional.

Parameter | Data Type | Description
:--- | :---  | :---
ignore_unavailable | Boolean | Defaults to `false`. When set to `false` the response body will return an error if an index is closed or missing.
allow_no_indices | Boolean | Defaults to `true`. When set to `false` the response body will return an error if a wildcard expression points to indexes that are closed or missing.
expand_wildcards | String | Expand wildcard expressions for indexes that are `open`, `closed`, `hidden`, `none`, or `all`.
search_type | String | Set search type to either `query_then_fetch` or `dfs_query_then_fetch`.

## Request fields

The request body must contain at least one parameter.

Field Type | Description
:--- | :---  
id | Document or template ID.
requests | Set multiple search requests within the request field section.
ratings | Document relevance score.
k | The number of documents returned per query. Default is set to 10.
relevant_rating_threshold | The threshold at which documents are considered relevant. Default is set to 1.
normalize | Discounted cumulative gain will be calculated when set to `true`.
maximum_relevance | Sets the maximum relevance score when using the expected reciprocal rank metric.
ignore_unlabeled | Defaults to `false`. Unlabeled documents are ignored when set to `true`. 
template_id | Template ID.
params | Parameters used in the template.

#### Sample request

````json
GET shakespeare/_rank_eval
{
  "requests": [
    {
      "id": "books_query",                        
      "request": {                                              
          "query": { "match": { "text": "thou" } }
      },
      "ratings": [                                              
        { "_index": "shakespeare", "_id": "80", "rating": 0 },
        { "_index": "shakespeare", "_id": "115", "rating": 1 },
        { "_index": "shakespeare", "_id": "117", "rating": 2 }
      ]
    },
    {
      "id": "words_query",
      "request": {
        "query": { "match": { "text": "art" } }
      },
      "ratings": [
        { "_index": "shakespeare", "_id": "115", "rating": 2 }
      ]
    }
  ]
}
````

#### Sample response

````json
{
  "rank_eval": {
    "metric_score": 0.7,
      "details": {
      "query_1": {                           
        "metric_score": 0.9,                      
        "unrated_docs": [                         
          {
            "_index": "shakespeare",
            "_id": "1234567"
          }, ...
        ],
        "hits": [
          {
            "hit": {                              
              "_index": "shakespeare",
              "_type": "page",
              "_id": "1234567",
              "_score": 5.123456789
            },
            "rating": 1
          }, ...
        ],
        "metric_details": {                       
          "precision": {
            "relevant_docs_retrieved": 3,
            "docs_retrieved": 6
          }
        }
      },
      "query_2": { [... ] }
    },
    "failures": { [... ] }
  }
}
````