Process diagnosis is critical to the semiconductor industry. Early detection of faulty process steps can dramatically reduce business losses and accelerate the development cycle of new products. This article proposes a novel methodology called AC/ID (automatic classification/interactive diagnosis) that can isolate the root causes of yield loss by combining end-of-line tests (such as wafer sort test) with process history information. The authors have automated the AC/ID methodology in a software tool, ACID (Automatic Classification for Interactive Design), which is fully operational at industrial production sites.