Previous year DAX measure

[Anonymous]

Hi - I'm trying to create a measure that calculates the previous year's value i.e. for 2015 it would show 346 (6617-6271). 

I've tried SAMEPERIODLASTYEAR and PARALLELPERIOD with no success - the "year" column is a product of powerpivot auto generating separate date fields (month, quarter, year) based on a single date column in the data i.e. The "year" in the table below is actually ServiceDate(Year) in the field list and is based off a field called ServiceDate. Hoping someone can help out with this, thanks in advance!

Year                  Unique Patients 

2014	6,271
2015	6,617
2016	6,544
2017	6,118