Predictive validity of the strain index in manufacturing facilities

The Strain Index is a job analysis method for determining if workers are exposed to increased risk of developing distal upper extremity disorders. Its predictive and external validity was initially demonstrated in a pork processing plant. The purpose of this study was to evaluate its predictive validity in two manufacturing plants. While blinded to health outcomes, investigators analyzed the right and left sides of 28 single-task jobs using the Strain Index and classified them as "hazardous" or "safe" based on the Strain Index score. Subsequently, OSHA 200 logs were used to ascertain the occurrence of distal upper extremity disorders retrospectively. If at least one such disorder occurred on the right or left side during the prior three years, that side was classified as "positive." If no such disorder was reported during the prior three years, that side was classified as "negative." When comparing sides, symmetry between morbidity and hazard classification was required. When comparing jobs, such symmetry was not required. Evidence of association between the hazard classifications and the morbidity classifications for the 56 sides and the 28 jobs was evaluated using 2 x 2 contingency tables. For the sides, the association between hazard classification and morbidity classification was statistically significant with an empirical odds ratio of 73.2. The sensitivity, specificity, positive predictive value, and negative predictive value were 1.00, 0.84, 0.47, and 1.00. Similar results were noted for the jobs--the empirical odds ratio was 106.6, and the sensitivity, specificity, positive predictive value, and negative predictive value were 1.00, 0.91, 0.75, and 1.00. While these results provide additional evidence of the Strain Index's external validity and predictive validity, it should be noted that these jobs involved the performance of single tasks.