Audience: SharePoint Server 2013 and SharePoint Online search administrators.
 
Before you start:
To make changes to the ranking of search results, you need:
• A basic understanding of search in SharePoint 2013. 
• To know and understand the content that is returned in search results to judge how relevant a search result for a given query is.
• A running Search service application and Enterprise Search Center.
• Content in the search index.

Why is search result ranking important?
Whether you’re working with an Enterprise Search Center on premises, with SharePoint Online, or with a cross-site publishing solution, your search results will be ranked. In most cases, this default search result ranking should be just fine.

So, what’s in it for you and why are we blogging about this? Well, sometimes, you may want to influence the ranking of search results to make results even more relevant to your end-users. We recently published a set of articles on TechNet and MSDN that explain how you can change the ranking of search results and that will help you understand how search result ranking works in SharePoint 2013. 

How are search results ranked?
Search results are ranked using a ranking model. A ranking model calculates the position of a search result in the result set. There are several ranking models in SharePoint 2013 that automatically do this for you. So, usually, you don’t have to care about which ranking model is used for a query, or what it does exactly.

How can I influence the ranking of a search result?
You can influence the ranking of search results in a relatively simple and effective way using query rules, but you can also take the complex route by deploying a custom ranking model.

Using query rules to change search result ranking
If you’re not satisfied with the search result ranking that SharePoint 2013 provides, we recommend that you add query rules to optimize search result ranking for your search scenarios.

The good thing about query rules is that they’re available to a large range of search administrators. You can add query rules to the Search service application as a search administrator on premises, or as a tenant administrator in SharePoint Online. You can also add and (re)use query rules as a site collection administrator or site owner, both on premises and online.

For each query rule, you can influence the way you sort, rank and display search results. Each query rule consists of a query rule condition and a query rule action. Whenever a query matches a query rule condition, the query rule action that you specify in the query rule triggers. After you have entered a condition, you can specify to:

  • Add promoted results that always appear on top of the ranked search results.
  • Add a result block that shows particular search results as a group, and promote that block. 
  • Change the order of the search results. 
          - Sort by one or several managed properties
            When you sort results this way, you override the ranking model.
          - Apply dynamic ranking
            You can promote or demote results based on a query condition that you specify.

The section Influence the ranking of search results with query rules in the TechNet article Overview of search result ranking contains more details. Although most of the time you can use query rules to adjust ranking, be careful not to add too complex or too large a number of query rules to avoid increased query latency.

Using custom ranking models: if query rules don't do the trick
If it turns out that you can’t use query rules to achieve your goals, you can consider creating and deploying a custom ranking model. For example, you can create a custom ranking model to include custom managed properties in the search result ranking calculations.

Since creating and tuning a custom ranking model is complex and can have a huge effect on your search results, we recommend that you do not take this lightly. You can only create and deploy a custom ranking model on premises.

When you create a custom ranking model, you copy an existing SharePoint 2013 ranking model and edit that copy. Then, you should validate how good the custom ranking model is by running many queries and comparing the results you get with the new ranking model to the results you got with the previous ranking model. Once you're done creating and validating, you deploy your custom ranking model and tell the search system that it should use the new ranking model to rank all or some of your search results.

As with any ranking model that comes with SharePoint 2013, a custom ranking model calculates the position of a search result in the result set. A search result is considered relevant if it receives a high rank score. A high rank score is a specific numeric score that’s calculated by the search engine, using a ranking model. A ranking model is a list of one or more rank stages that contain a set of rank features. The ranking model defines how the search engine calculates the relevance rank using various factors, which are represented in the ranking model as rank features.

There are several ranking models available in SharePoint 2013. For more details, see Overview of search result ranking on TechNet. Most search results are ranked using the Default Search Model. Read Customizing ranking models to improve relevance in SharePoint 2013 on MSDN to learn more about the most important ranking features in the Default Search Model. This article also explains how to deploy a custom ranking model.

We hope that the articles give you an overview of how search result ranking works and how you can make changes to it. If you have any feedback or questions, please leave us a comment on this blog.

Tiago Pregueiro (PM) and Scarlet Bulterman (SharePoint IT Pro writer)

Additional information:
Overview of search result ranking in SharePoint Server 2013 - TechNet
Plan to transform queries and order results in SharePoint 2013 - TechNet
Manage query rules - TechNet
Create query rules for web content management in SharePoint Server 2013 - TechNet
Customizing ranking models to improve relevance in SharePoint 2013 - MSDN 
Manage query rules - Office.com