In this paper the formulae are developed to predict the approximation error resulted from the use of the energy level difference (ELD) method. The predicted approximation error can then be used to assess the feasibility of the application of the ELD method. The effects of different parameters on the approximation error are analysed and discussed. Two practical applications are presented. Both numerical and experimental results showed that the ELD method could give identical results (from experimental point of view) as other existing methods as long as alphai and beta(i) are small. (C) 2003 Elsevier Science Ltd. All rights reserved.