I want to calculate the rate of change/growth for a specific product's trade balance over time. In 1996 the trade balance was positive (more exported than imported). By 2006 it turned negative (more imported than exported) and was still so in 2008. What is the rate of change from +1200 million in 1996 to -7500 million in 2008? Compound growth calculations yield no result. How do I go about with the negative end value?