This was first discussed here:

In this context, "efficient" means "on the efficient frontier " while "inefficient" means "not on the efficient frontier." The construction of efficient shopping lists involves the construction of two efficient frontiers. One represents efficiency from an individual consumer's point of view. The other represents the pub wan movement's point of view. It is assumed that the individual consumer wants the maximum bang (or other desireable quantity, and/or the minimum amount of undesireable quantities) for the minimum buck (price). This minimization and maximization stuff constitutes the maxhi schema. It is assumed that the pubwan movement wants opportunities to fill in informational gaps in its database, and take (vicariously, by way of its volunteers) "field trips" to sites that are underrepresented in the database. The movement frontier therefore reflects a preference for items about which little or nothing is known (to pubwan) over items pubwan knows much about. Likewise for vendors about which pubwan knows little. Shopping with a shopping list that is movement-efficient is a way in which one can contribute consumer surplus to the pubwan movement. (see pubwan participant)

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