An international nonprofit organization finances medical research. The majority of its revenue and support comes from fund-raising activities, investments, and specific grants from an initial sponsoring corporation. The organization has been in operation for over 15 years, and has just finished a major fund-raising event that raised $500 million for the current fiscal period. The following are selected data from recent financial statements (in millions of dollars).

	Current Year	Past Year
Revenue	$500	$425
Investments (average balances)	210	185
Investment income	16	20
Administrative expense	10	8

A financial analyst wants to determine if the change in investment income during the current year was due to changes in investment strategy, changes in portfolio mix, or other factors. Which one of the following techniques should be used?