A common experience when shopping is to see price discounts expressed in percentage terms. "All items are 25% off". That's easy enough to understand. How about situations where you see signs that say "Take a further 15% off at the register"? This is where complications arise. If an item costs $100 and you see these two signs what do you think is the final discounted price of the item? Multiple percentage changes are often used by stores and for good reason. Recent research shows that consumers are not very good at calculating multiple percentage changes and in fact make predictable mistakes. The researchers show that these mistakes can be rectified in certain ways and that there is a clear economic cost to consumers when such mistakes are made.