Effect of Changes in the Sequence of Assembly Operations on Error Rates: A Case Study From the Car Manufacturing Industry

Human operator-induced assembly errors affect the quality of car manufacturing. Understanding the factors influencing assembly errors is critical for quality improvement. The sequence of assembly operations, a factor markedly affecting cognitive load, remains largely understudied. We aimed to assess the effect of changing the sequence of assembly operations on error rates through four field experiments conducted in a car manufacturing plant. The parts (and errors) under study were child lock labels (missing labels), sunroofs (missing bolts), windshield wiper arms (loose bolts), and armrests (wrong selection). Parts were chosen based on data from quality records, and they represent different scenarios regarding the sequencing of assembly operations. Minitab was used to conduct the statistical test for two proportions at a significance level of 0.05. The experiments ran for a period varying from 9 to 25 weeks (22292 to 138456 cars). All four experiments exhibited significant differences in the proportion of errors. MODAPTS cycle-time calculations revealed no negative effect of assembly sequence variations on productivity. The study findings show that changing the assembly operation sequence can reduce the error rates, possibly due to the intermediary effect in reducing the operator’s cognitive load. Overall, realizing quality improvement requires optimizing the assembly operation sequence in terms of time and productivity while considering its possible impact on error rates.