Savvy grocery buyers compare prices per unit

“Ellen and Edward Barbagallo are standing in the snack aisle of Target deliberating over which bag of pretzels to buy — the 16-ounce or the 10-ounce. Flavor doesn’t have anything to do with this decision. Neither does brand, since both options are Snyder’s. It’s a math question. Ellen Barbagallo digs through her purse and takes out her calculator.”