A method of determining frequencies for use in performing impedance measurements, the method including: a) obtaining impedance data from a sample population; b) determining estimates for parameter values of parameters (R i, R e, C) in an impedance model that effectively models the electrical behaviour of biological tissue, the parameter values representing an impedance response for a subject, the estimates representing the measured responses of subjects of the sample population, characterised in that the method comprises determining estimates for the parameter values by: i) determining a residual variance model having a predetermined distribution, and, ii) determining estimates for the parameter values using the residual variance model, the impedance data and said impedance model and wherein the method further includes using the estimated parameter values to determine a design by: i) constructing a number of constructed impedance models using different estimated parameter value and said impedance model ii) determining a Fisher information matrix for each of the constructed impedance models; and iii) optimising the product of the determinants of the Fisher information matrices to determine a design representing frequencies for use in impedance measurements; and (e) using the design to determine frequencies for use in impedance measurements. a) obtaining impedance data from a sample population; b) determining estimates for parameter values of parameters (R i, R e, C) in an impedance model that effectively models the electrical behaviour of biological tissue, the parameter values representing an impedance response for a subject, the estimates representing the measured responses of subjects of the sample population, characterised in that the method comprises determining estimates for the parameter values by: i) determining a residual variance model having a predetermined distribution, and, ii) determining estimates for the parameter values using the residual variance model, the impedance data and said impedance model and wherein the method further includes using the estimated parameter values to determine a design by: i) constructing a number of constructed impedance models using different estimated parameter value and said impedance model ii) determining a Fisher information matrix for each of the constructed impedance models; and iii) optimising the product of the determinants of the Fisher information matrices to determine a design representing frequencies for use in impedance measurements; and (e) using the design to determine frequencies for use in impedance measurements. i) determining a residual variance model having a predetermined distribution, and, ii) determining estimates for the parameter values using the residual variance model, the impedance data and said impedance model i) constructing a number of constructed impedance models using different estimated parameter value and said impedance model ii) determining a Fisher information matrix for each of the constructed impedance models; and iii) optimising the product of the determinants of the Fisher information matrices to determine a design representing frequencies for use in impedance measurements; and (e) using the design to determine frequencies for use in impedance measurements.