New to Microsoft Excel? Looking for a tip? How about a tip so mind-blowingly useful as to qualify as a magic trick? You're in luck. In this MS Excel tutorial from ExcelIsFun, the 611th installment in their series of digital spreadsheet magic tricks, you'll learn how to create an array formula to summarize year over year sales using the SUMPRODUCT and YEAR functions. Also see how to calculate a formula for percentage change.