Design of cost optimal experiment plans on the basis of a preposterior analysis is discussed. In particular the planning of on-site response measurements on offshore structures in order to update probabilistic models for fatigue life estimation is addressed. Special emphasis is given to modelling of uncertainties in the transfer function. An example is presented in which the optimal number of response spectra measurements is sought. ; Design of cost optimal experiment plans on the basis of a preposterior analysis is discussed. In particular the planning of on-site response measurements on offshore structures in order to update probabilistic models for fatigue life estimation is addressed. Special emphasis is given to modelling of uncertainties in the transfer function. An example is presented in which the optimal number of response spectra measurements is sought.