Context The key for effective problem prevention is detecting the causes of a problem that has occurred. Root cause analysis (RCA) is a structured investigation of the problem to identify which underlying causes need to be fixed. The RCA method consists of three steps: target problem detection, root cause detection, and corrective action innovation. Its results can help with process improvement. Objective This paper presents a lightweight RCA method, named the ARCA method, and its empirical evaluation. In the ARCA method, the target problem detection is based on a focus group meeting. This is in contrast to prior RCA methods, where the target problem detection is based on problem sampling, requiring heavy startup investments. Method The ARCA method was created with the framework of design science. We evaluated it through field studies at four medium-sized software companies using interviews and query forms to collect feedback from the case attendees. A total of five key representatives of the companies were interviewed, and 30 case participants answered the query forms. The output of the ARCA method was also evaluated by the case attendees, i.e., a total 757 target problem causes and 124 related corrective actions. Results The case attendees considered the ARCA method useful and easy to use, which indicates that it is beneficial for process improvement and problem prevention. In each case, 24–77 target problem root causes were processed and 13–40 corrective actions were developed. The effort of applying the method was 89 man-hours, on average. Conclusion The ARCA method required an acceptable level of effort and resulted in numerous high-quality corrective actions. In contrast to the current company practices, the method is an efficient method to detect new process improvement opportunities and develop new process improvement ideas. Additionally, it is easy to use.