Watson retrieve&rank methods rank VS search and rank

Question

In Retrieve&Rank service documentation there are 2 methods about results ranking

• Rank: Returns the top answer and a list of ranked answers with their ranked scores and confidence values (http://www.ibm.com/smarterplanet/us/en/ibmwatson/developercloud/retrieve-and-rank/api/v1/?node#rank)
• Search and rank: Return reranked results for your query. The request is similar to the Search Solr standard query parser method (http://www.ibm.com/smarterplanet/us/en/ibmwatson/developercloud/retrieve-and-rank/api/v1/?node#query_ranker)

What are the differences? what "custom feature" means on rank method? when i need to use first method? and when the second method?

Answer 1

With the first, you're providing a question and a list of answers, and you're asking the service to rank the answers - to sort them in order of relevance based on the feature scores (that you also provide).

With the second, you're providing a question, and you're asking the service to do a Solr search to retrieve answers and then rank them in order of relevance.

The second is the most commonly used method - asking the service to do the search and to sort the responses ('retrieve and rank').

You'd use the first rank-only method if you want to provide the answers to be sorted, rather than use the Solr search to do that. Or if you wanted to modify the feature scores that are used to do the ranking.

There is a good description of that second part (using custom feature scores) here : https://medium.com/machine-learning-with-ibm-watson/developing-with-ibm-watson-retrieve-and-rank-part-3-custom-features-826fe88a5c63