Calculation (Formula) on report will change every year - preserve historical date

[crimedog]

so I have a company where the bonus amount for a calculation can change quarterly - if a person accomplishes 50-100% of plan they get that % of their bonus amount.


I have that working on a variable detail DB where the historical data is correct for the report.
i.e. if I want to look at January - the report looks at the requested date: January and calculates using the bonus number from the last update made before January (year is also factored in)
so: January 2014 if they make 50% of plan and their bonus is $100 this month - they receive $50

Good - no problem

NOW:
Every year the formula on the report Could Change - so next year if the person makes 50-100% of plan and 30% of secondary plan - they get 30%(% of Bonus)
so now: January 2015 if they make 30% of secondary plan and 50% of plan with $100 bonus the report would give .30*(.50*100) = 15


I can change the calculation on the report - BUT then how would I go back and accurately show what they got in January 2014

Would it require a different report per year?

[ItsMe]

I would create a table to log bonuses. I would include columns to represent variables, like percentage of plan and bonus amount. So maybe, four columns including the actual amount paid and a PK.