69 Chapter 3 who report a mean FSS of 2.3 ± 0.7 (n = 20) again the mean FSS score in the Norwegian control group is high. However, there may be an explanation for the high score in the control group as a Norwegian national study investigating fatigue in the general population, 19 concluded that the high FSS scores in the general population of Norway can be due to difficulties in translation of the US-English version of the FSS into Norwegian because of lack of the concept of fatigue in Norwegian language 19. A valid comparison between Norway, the Netherlands and the US regarding the FSS may therefore not be possible. A validation of the FSS in a Swiss control group is comparable to the Dutch and American results with a mean FSS score of 3.00 ± 1.08, (n = 454) 20. As such, we can conclude that the mean FSS score of our Dutch control group is comparable with the American and Swiss control groups and that our earlier conclusion that the severity of fatigue is increased in the Dutch OI cohort still holds true. Limitations of this study and further directions for research There is a low response rate (151/221 gave consent and 99/151 filled in the FSS) when looked at the initially approached patients. It is difficult to speculate why this could be the case but an important factor may be that with regard to consent as well as with regard to filling in the FSS, patients were only approached once and were not sent reminder(s). Biases are difficult to avoid as it may be that the people who felt that fatigue was influencing there life significantly, were more inclined to participate but it is also possible that these patients were limited by fatigue to participate in the study. As mentioned before, there are many scales to measure the nature, severity and impact of fatigue in a range of clinical populations and a limitation of the FSS is that it is a general questionnaire, and as such not specially developed for OI. The FSS however explores the severity of fatigue and is therefore suitable for initial screening in different clinical populations and can be used for longitudinal measurements which is important in assessing whether fatigue can increase or decrease over time and exploring possible modifiers of fatigue. Another limitation of our study lies with the control populations as both the Dutch control group and the US control group date from respectively 1999 and 1989 and trends in fatigue may change in the population over time. Lastly, we did not investigate any factors that influence fatigue in OI patients in our study, but this is an important direction for further research into fatigue in patients with OI as fatigue may influence QOL. Other factors have been reported as well 21. It is already known that the presence of pain,
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