The variability associated with composting can limit the ability to detect statistically significant differences between treatments. This study investigated the effect of a wastewater inoculum on the process dynamics and variability associated with temporal changes in temperature, moisture content, and effluent oxygen concentration, as well as spatial changes in temperature and moisture content. Statistical tests suggested that the inoculum had little effect on the above variables although the statistical power or the ability to detect statistical differences for small, medium, and large differences between treatments for temperature and moisture content was typically below 80% for the differences tested. Comparison of the temperature data showed that the inoculum was able to decrease the experimental variation, but had no positive effect on moisture content variability. Calculation of the statistical power associated with temperature data from a previous study showed that the statistical power found for this study was much higher than typical composting studies because of the decreased variability caused by the inoculum. (C) 2000 Elsevier Science Ltd. All rights reserved.