Rats worked under a fixed-ratio (FR) 45 schedule of reinforcement during 4-hr long sessions either in 16 15-min work periods (2 rats in Exps 1 and 3) or in a single work period (3 rats in Exps 2 and 4) while receiving varying amounts of external food. In Exps 1 and 2, a fixed amount of external food was provided in different conditions, whereas in Exps 3 and 4, both earned and total food intake were fixed to a daily maximum. Consumption and responding decreased with availability compared to nonavailability of external food and systematically declined with increasing amounts of external food in progressively open economies. The independence-quotient statistic was differentially sensitive to the "delay" to the external food. Discriminability enhanced the substitution effect of performance-independent food, resulting in improved efficacy of the statistic and the conditions defined along economic continuum.