How to write logic for Return on assets ( ROA)

Hi ,

I am working to develop different KPI ( Key performance Indicators )  . can any one help be to build following logic .

  Return on Assets = Net Income / Average total Assets .

                                          Average Total Assets = Total assets previous year / total assets current year .

Confusion here is how can we get current and previous year data at same time ?

Please suggest me can we use Dimension formulas or script logic ? . client is not interested in Excel 

Appreciates your inputs.

thanks,

bala.