Finding data associated with date

[dwatt]

I have records I keep year after year.
I want to obtain a particular field value from say year 2002 the Update field, and place that value in the PreviousYearValue for subsequent analysis to update records using calculated value....hope I'm making sense (see example table below).

Year Calculated PreviousYearValue Update
2002 5 0 5
2003 10 5 15
2004 25 15 40

Hope can be resolved
Thanks
David