DAX function to get previous row value grouped by another column

luckygirl Try this:

Previous Product Detail Column =
  VAR __Code = [ClientCode]
  VAR __Date = [CompletedDate]
  VAR __Table = FILTER( ALL( 'Table' ), [ClientCode] = __Code && [CompletedDate] < __Date )
  VAR __PrevDate = MAXX( __Table, [CompletedDate] )
  VAR __Result = MAXX( FILTER( __Table, [CompletedDate] = __PrevDate ), [ProductDetail] )
RETURN
  __Result



or

Previous Product Detail Measure =
  VAR __Code = MAX( 'Table'[ClientCode] )
  VAR __Date = MAX( 'Table'[CompletedDate] )
  VAR __Table = FILTER( ALL( 'Table' ), [ClientCode] = __Code && [CompletedDate] < __Date )
  VAR __PrevDate = MAXX( __Table, [CompletedDate] )
  VAR __Result = MAXX( FILTER( __Table, [CompletedDate] = __PrevDate ), [ProductDetail] )
RETURN
  __Result

See my article on Mean Time Between Failure (MTBF) which uses EARLIER: http://community.powerbi.com/t5/Community-Blog/Mean-Time-Between-Failure-MTBF-and-Power-BI/ba-p/339586.
The basic pattern is:
Column = 
  VAR __Current = [Value]
  VAR __PreviousDate = MAXX(FILTER('Table','Table'[Date] < EARLIER('Table'[Date])),[Date])

  VAR __Previous = MAXX(FILTER('Table',[Date]=__PreviousDate),[Value])
RETURN
 ( __Current - __Previous ) * 1.