I think the question is with regard to Dashboard Studio.  From the "Search" dialog you can sample data and have it create a Classic Dashboard, where the sampleRatio parameter is populated.  However if you choose to create a dashboard from your search using Dashboard Studio, then the sampleRatio parameter is not carried over (v9.0.4).   Either it is a missing feature or missing documentation.  I find it interesting that even new Splunk documentation has samples using the old dashboard technology and not Dashboard Studio.  ... View more