With the new FAST search core, ranking has changed quite a lot from 2010. Newly published content on MSDN explains a bit more how rank is calculated and how you can change it.
As the O14 rank model is available in SharePoint 2013 (O15), I will try to outline some of the major differences you can expect to see regarding how results are ranked/sorted by default.
You can pull out the rank model xml yourself from both models using PowerShell.
$ssa = Get-SPEnterpriseSearchServiceApplication $owner = Get-SPenterpriseSearchOwner -Level ssa $o15 = Get-SPEnterpriseSearchRankingModel -SearchApplication $ssa -Owner $owner -Identity 8f6fd0bc-06f9-43cf-bbab-08c377e083f4 $o15.RankingModelXML > o15.xml $o14 = Get-SPEnterpriseSearchRankingModel -SearchApplication $ssa -Owner $owner -Identity 9399df62-f089-4033-bdc5-a7ea22936e8e $o14.RankingModelXML > o14.xml
Then it’s all a matter of comparing the models.
File formatsIn O14 file formats are given quite a lot of weight compared to O15. In descending order file formats are prioritized like this (items in red have negative weights):
HTML, DOC, PPT, TXT, XML, XLS, Messages, Image, ListItem
O15 on the other hand ranks file formats like this:
PPT, DOC, HTML, ListItem, Image, Message, XLS, TXT, XML
The weight range also varies. O14 uses 1.45 to –0.031, while O15 uses 0.68 to -1,29.
The most noticeable difference is that PowerPoint's will move quite high in the result list for SharePoint 2013, and Excel quite a bit down for O15 (like it did for FAST Search for SharePoint as well).
The important thing here is to think about what files are important to you. In my experience PowerPoint’s should at least move down below Word documents. If pages should rank above PowerPoint depends largely on your content. But you should have an opinion and try it out.
ProximityO14 also seem to put more emphasis on click distance and url depth (shorter links) compared to O15. And as far as I can tell, O15 will give improved relevance to documents where the query terms match exact on a title.
BM25FBoth models use BM25F as their main model. The easiest way to explain this model is that it will ranks a set of documents based on the query terms appearing in each document set in context to how many times the words appear in total in all documents and within each particular document. The formula is listed on the MSDN link at the top if you want to get into the details.
Both models seem similar, but there are weight differences, but it’s not easy to compare this without trying out queries for each model and see which rank module outputs the actual score. But that’s for another post.
FreshnessNeither model pays any attention to when a document was created, which it actually did in FAST Search for SharePoint. You can however add this to the rank model with an entry similar to the one below (copied from MSDN). Quite useful when you have content spanning decades to surface newer and often more valid content.