I like to comparison shop - to calculate whether it is a better deal to buy a small at "10% off" versus a medium "15 oz at the 12 oz price", versus a large "Buy 3 get 1 free" versus an extra large at "Low, everyday regular price."

And it is not a coincidence that the small one has a shelf tag reading "cost per ounce", the medium "cost per milligram" the large "cost per serving" and the extra large "cost per each".

I whip out my iPhone calculator and do the math, slowly realizing that they are all within a tenth of a cent of being the same!

The same tactic is used when trying to compare brands.