SummaryMeasurements taken on a continuous process (e.g. production of chemicals, ore processing, steel production) often exhibit autocorrelation. Such correlation between successive measurements can be avoided only by taking measurements at widely separated times, but this delays the results. We discuss the impact of this correlation on experimental designs which for a continuous process must include the sequence in which treatment combinations are applied and the spacing between samples. The variogram of the process is a convenient Summary of the correlation. Formulae for the variance of the mean of measurements taken on a continuous process and of contrasts calculated from such measurements are simple expressions in terms of the variogram. These formulae allow a simple approach to experimental designs that avoid unnecessary delays.