The correct formula to calculate the percent difference in sales relative to the same quarter the previous year is (SUM([Sales]) - LOOKUP(SUM([Sales]), -4)) / (LOOKUP(SUM([Sales]), -4)). This formula finds the difference in sales between the current quarter and the same quarter in the previous year using LOOKUP(SUM([Sales]), -4), which looks up the sales figure from four quarters ago. The difference is then divided by the sales from the same quarter in the previous year to give the percent change. The results in the 'Calculation' field in the dataset confirm that this is the correct approach.