I ran across this press release about an study published in The Journal of Marketing that blames "bad math" for customers not recognizing which package was the best bargain. Presented with a percentage off a package (arguably harder to then calculate unit price) with a percentage more added to a product, customers were fooled as to which item was actually the better deal.
Now, I'm not always the best at math and sometimes it takes me a moment to do the rough calculations in my head -- particularly when I have to convert say ounces to liters in the soda aisle since the shelf tags are not generally uniform with measurement to make comparison a two second process. However, if you have a smart phone, there's an app for that (both Apple and Android) and Google will even do a conversion if you type it in the search bar. When there are pennies that need pinched, always do the math to make sure you're getting the best deal possible. That clearanced item may not be the best deal after all!