One attractive feature of optimum design criteria, such as D-and A-optimality, is that they are directly related to statistically interpretable properties of the designs that are obtained, such as minimizing the volume of a joint confidence region for the parameters. However, the assumed relationships with inferential procedures are valid only if the variance of experimental units is assumed to be known. If the variance is estimated, then the properties of the inferences depend also on the number of degrees of freedom that are available for estimating the error variance. Modified optimality criteria are defined, which correctly reflect the utility of designs with respect to some common types of inference. For fractional factorial and response surface experiments, the designs that are obtained are quite different from those which are optimal under the standard criteria, with many more replicate points required to estimate error. The optimality of these designs assumes that inference is the only purpose of running the experiment, but in practice interpretation of the point estimates of parameters and checking for lack of fit of the treatment model assumed are also usually important. Thus, a compromise between the new criteria and others is likely to be more relevant to many practical situations. Compound criteria are developed, which take account of multiple objectives, and are applied to fractional factorial and response surface experiments. The resulting designs are more similar to standard designs but still have sufficient residual degrees of freedom to allow effective inferences to be carried out. The new procedures developed are applied to three experiments from the food industry to see how the designs used could have been improved and to several illustrative examples. The design optimization is implemented through a simple exchange algorithm.We shall refer to this as the DP.α/ criterion. In this paper, we shall use α = 0:05 for illustration and refer to the criterion simply as DP, but the required confidence level should be considered carefully for each experiment. Despite the above quotation, Kiefer (1959) did not suggest this additional step, since he did not separate lack of fit from pure error.Similarly, D S -optimality is intended to minimize the volume of a joint confidence region for a subset of p 2 of the parameters by minimizing |.M −1 / 22 |, where M = X X and .M −1 / 22 is the portion of its inverse corresponding to the subset of the parameters of interest. To take account of pure error estimation correctly, the .DP/ S criterion is to minimizeThis criterion should be used, for example, when a major objective of the experiment is to compare the first-order model with the second-order model. Then the higher order terms will form the subset and minimizing the volume of a confidence region for them will be equivalent to maximizing the power of a test for their existence. Note that if the parameters of interest are the treatment parameters and the nuisance parameter(s) is or are the intercept or the int...