We thank Gerr and Fethke for their response to our remarks on their editorial on self-reported versus objectively recorded computer times [1]. Gerr and Fethke continue to disregard the consistent evidence that objectively recorded computer times are much more accurate and valid than self-reported computer times when compared to external “gold” standards [2,3]. They suggest that self-reported computer times may be superior to objectively recorded computer times in capturing biologically relevant risk factors for musculoskeletal disorders. Their basic argument is that musculoskeletal disorders seem to be related to self-reported but not to objectively recorded computer times. According to Gerr and Fethke this discrepancy could be due to error in self report, differences in the kind of exposure information captured, or both. They blame us that our “claims of methodological objectivity and validity … do not address this fundamental question”.

However, we did address this question [3]. We studied if the differences between self-reported and objectively recorded computer times depended on the level of computer times, variation in computer times, level of arm pain, psychosocial work characteristics, age, gender, and personality characteristics. Concerning work characteristics, self-reported computer times increased with a high degree of variation between weeks; and a one hour increase in weekly computer time was overestimated, especially at low levels of computer time. In our opinion these findings reflect self-report biases, rather than self-report “captures” of biologically relevant information. We found no musculoskeletal health effects of computer work speed, sustained activity or micropauses or their interactions with computer times [2,4]. Of course, other factors may also be considered and our findings should be replicated.

Gerr and Fethke are concerned about the generalisability of the NUDATA results with reference to our objective median exposure times [2,4]. However, these are without any relevance to generalisabilty. The NUDATA cohort was designed to include persons with low as well as high computer work hours in order to examine internal exposure-response relations. Therefore, it is the representation of variation in computer work hours that counts, not means or medians. These problems have been thoroughly discussed in our previous publications [2,4].

Sigurd Mikkelsen
Johan Hviid Andersen

References

1. Gerr F, Fethke N. Ascertaining computer use in studies of musculoskeletal outcomes among computer workers: differences between self-report and computer registration software. Occup Environ Med 2011; 68: 465-66.

2. Mikkelsen S, Lassen CF, Vilstrup I, et al. Does computer use affect the incidence of distal arm pain? A one-year prospective study using objective measures of computer use. Int Arch Occup Environ Health 2012;85:139-52.

3. Mikkelsen S, Vilstrup I, Lassen CF, et al. Validity of questionnaire self-reports on computer, mouse and keyboard usage during a four-week period. Occup Environ Med 2007; 64:541-7.

4. Andersen JH, Harhoff M, Grimstrup S, et al. Computer mouse use predicts acute pain but not prolonged or chronic pain in the neck and shoulder. Occup Environ Med 2008;65 :126-31.

Conflict of Interest: None declared