OBJECTIVES: The concept of efficient frontier, given a series of cost-effectiveness estimates for different levels of programs, plays an important role in the incremental analysis. The purpose of the study is to exploit a theoretical aspect of the incremental cost-effectiveness ratios (ICERs) in the context of the efficient frontier curve, and then to identify the upper and lower limits that bound the ICER, considering a potential application of the limits for the pricing rule based on the ICER. METHODS: Let two points be PA(Ea, Ca) and PB(Eb, Cb) for programs A and B, respectively, on the E-C plane representing a set of effectiveness and cost. Theoretical developments were undertaken to find a solution for the question on the boundaries, supposed there exists a concave graph of the efficient frontier curve, C=f (E), directing upward from zero on the E-C plane. Model calculation was performed, for an example, when the curve is a quadratic function estimated as C=pE2+qE+r (p, q, and r: constants). RESULTS: The interval with the derivatives:(f '(Ea), f '(Eb)) on C=f (E), respectively, at the points PA and PB was identified as a solution, called ‘operative interval,' which describes reasonable lower/upper boundaries of the ICER, considering the configuration of the graph. When the efficient frontier curve is quadratic, the width of the interval was estimated as 2p(DE). Namely, the width is variable, not constant, depending on both p and DE. Furthermore, the new benchmark identified four possible scenarios for making a decision to accept the ICER compared with a threshold of willingness-to-pay. CONCLUSION: The concept of the operative interval of an ICER has been introduced, and it suggested potential usefulness for assessing the acceptability.