When you were younger, you may have wondered why gas had odd prices like $1.09.9 and your parents likely told you it's because $1.09.9 looks cheaper than $1.10.   Stores love prices that end in $.99 for that reason.

But numbers work the other way as well - a larger number looks better, even if its value is equivalent to a smaller number scaled differently.   Writing in Journal of Consumer Research, Mario Pandelaere and Christophe Lembregts of Ghent University,  along with Barbara Briers of Tilburg University,  note that higher numbers seem to represent bigger quantities to many consumers. This “unit effect” is the reason why consumers perceive a bigger difference between ratings 90 and 95 out of 100 than they do between a 9 or 9.5 out of 10.

And the difference between an 84-month and a 108-month warranty looks larger than the difference between a 7-year and a 9-year warranty, though the actual differences are the same.

“Qualitative information can usually be specified in alternative units,” the authors write.  “In many cases, however, the specific unit in which information is described is arbitrary.  For instance, product quality ratings may be expressed on a scale from 0 to 10 or on a scale from 0 to 100.   People typically fail to realize that the unit of quantitative information is arbitrary. They just focus on the number of scale units used to express a certain difference.”

But there may be an upside to that, at least in healthy food choices.   In an additional study, participants exiting the lab were offered the choice between a complimentary apple or a Twix® bar. The energy content of these two choices was either expressed in Kilojoules (247 for the apple versus 1029 for the Twix®) or Kilocalories (59 for the apple versus 246 for the Twix®). “Participants more often chose the apple when the energy content was expressed in Kilojoules than in Kilocalories as the former difference (782 Kilojoules) looks much bigger in the latter one (187 Kilocalories).

But the effect was not replicated when people pay close attention to specific attribute information or when people are reminded of the arbitrary nature of the unit in which information is expressed.

Citation: Mario Pandelaere, Barbara Briers, and Christophe Lembregts. “How to Make a 29% Increase Look Bigger: The Unit Effect in Option Comparisons.” Journal of Consumer Research: August 2011 (published online February 1, 2011).