Here's my attempt at creating that Sample Report via a Formula field from today's #MVP Office Hours

 call.  

I created a plain vanilla Auto-Number field, that just assignes a number sequentially as they are creaed.  So it's not "true randomization", but close enough.  

Then I created a custom Formula(Number) field  

Field Name = Nth (or whatever you want)   

Datatype = Formula       

Result = Number( 0 decimals ) 

Formula = 

VALUE(RIGHT( Auto_Number__c , 1))

So now I can build a Report and add my Nth field to my report filter and say Nth equals 1, and the report will give me every 10th record created (10% sample size)