Computer use was measured based on the response to the following open question: "How many hours per week do you usually use a computer?" The responses were categorized according to the sample distribution into three groups as follows: (i) 3.5 hours or less per week (corresponding ≤1/2 hour/day, N = 105); (ii) 3.6-13.99 hours per week (corresponding < 2 hours/day, N = 164); and (iii) 14 hours or more per week (corresponding ≥2 hours/day, N = 148). Those who did not use computers at all (N = 19) were excluded from the analysis.
The questionnaire included questions about two kinds of pain: musculoskeletal pain and computer-associated musculoskeletal pain. The latter question assessed the participants' own perception whether computer use had caused pain to them. Musculoskeletal pain was assessed with the question: "During the past half year, have you had some of the following symptoms and how often?" After the question six symptoms were mentioned (neck-shoulder, low back, head, eyes, hands, and fingers or wrists) and for each of them the following alternatives were given: a) seldom or not at all, b) about once a month, c) about once a week, and d) almost daily. Participants were asked to evaluate the level of inconvenience to everyday life due to pain using the VAS . The 100-mm vertical VAS scale was marked at one end as "not inconvenient at all" and at the other end as "very inconvenient indeed". The level of the inconvenience caused by pain was assessed using the following question: "If you have musculoskeletal symptoms, how much inconvenience do they cause you in your everyday life?" The respondents were asked to make a mark on the line to indicate the level of inconvenience at the pain sites. The means for inconvenience were: head 20.7 mm (range 0-90 mm); neck or shoulders 16.1 (0-91) mm; low back 12.7 (0-76) mm; eyes 7.6 (0-77) mm; and hands, fingers and/or wrists 6.7 (0-68) mm.
Computer-associated musculoskeletal pain was assessed with the question: "Using a computer may cause symptoms (pain, aches, discomfort) in the following anatomic locations in the body. Have you experienced such symptoms?" The symptoms in neck-shoulder, low back, head, eyes, hands, and fingers or wrists, were named with the response alternatives a) not at all, b) about once a month, c) about once a week, and d) almost daily. Participants were asked to evaluate the pain intensity on the VAS-scale by the question: "If you have had computer-associated symptoms, how severe have they been?", and to make a mark on the line to indicate the intensity of each symptom. Here the endpoints were "no pain at all" and "very severe pain". The means of pain intensity were: head 15.3 mm (range 0-99 mm); neck or shoulders 15.1 (0-88) mm; eyes 10.0 (0-100) mm; low back 10.0 (0-85) mm; and hands, fingers, and/or wrists 5.3 (0-71) mm.
The VAS scores for pain intensity and level of inconvenience were each stratified into 4 groups. For pain intensity, VAS scores less than 5 mm were recorded as 0 (no pain), according to Hunfeld et al., who consider only a score of 5 mm or above to indicate the presence of pain [15, 21, 22]. For pain intensity and level of inconvenience symptom groups, scores of 51 mm or more were considered to indicate severe pain or severe inconvenience to everyday life; 26 to 50 mm indicated moderate pain or moderate inconvenience to everyday life; and 5 to 25 mm indicated mild pain or mild inconvenience to everyday life, respectively. Because of the small number of cases with severe pain and severe inconvenience to everyday life, the categories "severe" and "moderate" were combined into one variable "moderate/severe pain" and "moderate/severe inconvenience to everyday life".
New variables were formed by combining groups according to pain occurrence and pain intensity or level of inconvenience, as follows: "intensity of computer-associated pain" for computer-associated pain and their VAS categories, and "level of inconvenience to everyday life caused by musculoskeletal pain" for musculoskeletal pain and their VAS categories. These alternative groups were: a) seldom/no pain, no inconvenience to everyday life, b) mild pain, mild inconvenience to everyday life, and c) moderate/severe pain, moderate/severe inconvenience to everyday life. Those who reported having no or rare pain, but who evaluated their pain as mild, were classified as category b.