Irradiation affects numerous physiological processes within cells and tissues and can lead to damage or death. If the damage is not too severe, cells have the ability to repair and regenerate. Many small injuries are repaired more easily than ones causing extensive damage and, consequently, tissues typically respond differently to one large dose of radiation than to many small doses, separated in time. In the radiotherapy of tumors, the choice of the fractionation regimen of dose over time is therefore as crucial as the total radiation dose. The interdependence between total dose, fractionation regimen, and radiation effect has been described mathematically with various isoeffect relationships. These relationships appear to be fundamentally distinct and have been considered unrelated; some even claim that one class of isoeffect relationships is appropriate whereas other relationships are rather useless. We examine how alternative isoeffect models relate to each other and test the reliability of estimating parameter values of one model from the other.