American Grocery Shoppers

Thus the academic solution is that the 28 oz. can is the better buy.

However, grocery shoppers rarely calculate unit price on their own, especially this precisely, when making best buy comparisons and often do not use store-provided unit price information. For example in the above situation, shoppers might first round off the prices and then mentally double the smaller can to get 29 oz for $4 dollars as opposed to 28 oz. for $3.60. This means that the extra ounce in two of the smaller cans would cost 40¢. Even without calculating unit prices, this 40¢ is clearly too large. Again the conclusion is that the 28 oz. can is the better buy.

At the same time, other information is often more relevant than unit price. For example, 14.5 oz. cans may closer approximate the quantitiy of tomatoes that the grocery shopper expects to use at one time. In this case, purchasing tomatoes in the 14.5 oz size, eliminates the need to open a 28 oz. can, use about half of it in meal preparation and then store the remaining half. For many shoppers, storing the remaining half often entails the risk of not using the tomatoes before they go bad while sitting in the refirgerator.

References

• Murtaugh, M. (1985). The practice of arithmetic by American grocery shoppers. In The Social Organization of Knowledge and Practice: A Symposium, Anthropology & Education Quarterly, 16, 171-213
• Borasi, R. (1998). Rethinking the nature of mathematics as a humanistic discipline. [website] http://www.rochester.edu/radiate/C/c1b_math.htm. [Accessed 19 March 2005]. Excerpted from R. Borasi (1992). Learning mathematics through inquiry. Portsmouth, NH: Heinemann (pp.158-164)
• Sternberg, R., (2000). Book Smart or Street Smart? [website] http://www.fathom.com/feature/122136/. Excerpted from Sternberg, R., Forsythe, G., Hedlund, J., Horvath, J., Wagner, R., Williams, W., Snook, S., & Grigorenka, E. (2000) Practical Intelligence in Everyday Life Cambridge: Cambridge University Press.