What is in-store slack? Consumers often plan for unplanned purchases

Those unplanned grocery purchases may not be so unplanned after all. According to a new study in the Journal of Consumer Research, shoppers often expect to buy a certain number of unplanned items, and most have a pretty good idea of how much they'll spend on them.

Authors Karen M. Stilley, Jeffrey Inman (both University of Pittsburgh), and Kirk L. Wakefield (Baylor University) say that consumers have spending expectations for grocery shopping trips, called mental budgets, and those budgets typically leave room for unplanned purchases. The authors call this "in-store slack."

The researchers conducted a field study at several grocery stores in Texas. They asked shoppers what items they planned to purchase, how much they expected to spend on the planned items, and how much they expected to spend on the total trip. After shopping, participants provided their receipts and answered questions about themselves and the experience. More than three-fourths of the participants included room for unplanned purchases.

"Shoppers in the study indicated that they employ this strategy both because they anticipate 'forgotten needs' as well as because they realize that they will encounter 'unplanned wants'—with some respondents even explicitly indicating that they expected to make impulse purchases," the authors write. Consistent with prior studies, the shoppers were remarkably accurate when predicting how much they would spend. The average budget deviation (actual spending minus planned spending) was only $0.47.

How does in-store slack affect household budgets? The impact of in-store slack on budget deviation depended on how many aisles the shopper visited and the shoppers' level of impulsiveness. "Less-impulsive individuals who shop most aisles tend to spend the money available from in-store slack, but don't exceed their overall budgets. In contrast, in-store slack leads to overspending for highly impulsive individuals who shop most aisles," the authors explain.

"For the majority of consumers, having in-store slack appears to be a rational way to use the store to cue needs and preserve self-control," the authors write. "Highly impulsive individuals may want to consider planning as many purchases in advance as possible."