Approximately 700 errors in four commercial measuring-control software products were analyzed, and the cause-effect relationships of errors occurring during software development were identified. The analysis method used defined appropriate observation points along the path leading from cause to effect of a software error and gathered the corresponding data by analyzing each error using fault tree analysis. Each observation point's data were categorized, and the relationships between two adjoining points were summarized using a cross-indexing table. Four major error-occurrence mechanisms were identified; two are related to hardware and software interface specification misunderstandings, while the other two are related to system and module function misunderstandings. The effects of structured analysis and structured design methods on software errors were evaluated.>