Background & Objectives: Assessment of physical risk factors related to musculoskeletal disorders is performed by different methods including observational methods. Validity of these methods are important in workplaces. The purpose of this study was to investigate the reliability of the Washington States ergonomics checklist as an observational method.

Methods: This descriptive-analytical study was performed on 50 workers and three tasks in a mattress manufacturing industry. The checklist completed by first group (non-specialists) and second group (ergonomic specialists) in the selected tasks. After assessment, the coefficient of agreement between observers with kappa value was analyzed using SPSS (version 16) software.

Findings: The first group of observers disagreed about the risk factors of hand force related to caution zone jobs checklist and awkward posture related to hazard zone jobs checklist and kappa value were 0.349 , 0.360 and 0.847, respectively. In the second group of observers, kappa values were 0.847 and 0.719 respectively about awkward posture and hand force risk factors.

Conclusion: Findings of this study showed that reliability of this method had been weak to good according to calculated kappa values between observers in the first group and good to excellent in the second group and there is possibility error in comparison between the two groups, which might be due to nonspecific nature of this tool and lack the necessary skills of the observers.