The share of unplanned purchases is an important measure for marketers. For instance, marketers need to know if purchases are planned or unplanned to allocate their marketing resources efficiently. Furthermore, the share of unplanned purchases could be seen as an indicator of how efficient marketers are in communicating with their customers in the stores (even if the in-store marketer is not the only one influencing in- store decision making.) A frequent assumption in the marketing literature is that the share of unplanned purchases increases with the size of the purchase. This paper questions this assumption by approaching the issue from a perspective that views the shopping trip as either a (more or less) contingency-dependent construction or as the execution of a well-defined behavior. Larger (major) trips are hypothesized and found to be more well defined, whereas smaller (fill-in) trips are found to be largely contingency- dependent constructions.