Defining the linear portion of a sigmoid-shaped curve: bend points

Formulae are provided that define the 'bend points', the beginning and end of the essentially linear dose-response region, for the four-parameter logistic model. The formulae are expressed in both response and dose units. The derivation of the formulae is shown in order to illustrate the general nature of the methodology. Examples are given that describe how the formulae may be used while planning and conducting bioassays. Copyright (C) 2003 John Wiley Sons Ltd.