Background: MUSIC-Norrtälje study is a case-referent study, the aim of which is to find risk and health factors for low back and neck/shoulder disorders. In this part of the study, the interview technique and the self-administered questionnaire used for assessment of physical loads are described and the inter-method reliability of parts of the self-administered questionnaire is estimated. The distribution of exposure levels in a general population is also described.
Methods: The study period was three years from November 1993 to November 1996, and the study subjects totaled 2,480 persons (813 female and 610 male referents, 380 female and 315 male low back cases, 252 female and 106 male neck/shoulder cases). The interview concerned "a typical working day" during the preceding 12 months and comprised assessment of energy expenditure, work postures, and manual materials handling for work and leisure time, including regular sport activities. The self-administered questionnaire comprised 18 questions, each covering 5 different points of time: right now, 5, 10, 15, 20 years ago. The answers to eight of the questions about current conditions were compared to corresponding interview responses. The interview was considered as the "gold standard."
Results: Ninety-eight percent of the subjects completed the interview without any great difficulties. According to the interview, the distributions of different exposure levels were generally positively skewed, i.e., the frequency of highly exposed subjects was low in the study base. The correlation between interview and questionnaire responses among the referents was high for time spent "sitting at work" (r = 0.82), "VDU work" (r = 0.87), and work related "motor vehicle driving" (r = 0.80). The correlation was moderate for work-related "hands above shoulder level" (rs = 0.63), and "hands below knee level (trunk flexion)" (rs = 0.66). The correlation was lower for leisure time activities such as "domestic work" (r = 0.55), "time for own activities" (r = 0.39), and "sitting during leisure time" (r = 0.38). Subjects seeking care for low back or neck/shoulder disorder estimated equally correctly or not, as had the referents. However, non-differential misclassification was present in all questions, which will attenuate observed estimates of the relative risk.
Conclusions: Even though interview data are preferable, questionnaire data may be useful for assessing well-defined work tasks and for "sitting at work."