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Labbe et al. [2012] examined the effect of methylphenidate in patients with attention deficit/hyperactivity disorder with special reference to the latrophilin 3 genes. They used accelerometer, named Actiwatch® (Mini-Mitter Co., Bend, OR), to monitor objective and quantitative wrist movement. Although the study design and evaluation method are valid and appropriate, there is a caution on their use of Actiwatch®.

The sampling frequency of the Actiwatch® was set at 32 Hz and the sensitivity at 0.05 g. They selected 15 sec interval as one epoch, and the number of body movements was calculated by summing the activity count. As a standard procedure, sleep/wake judgment was then made by using simple algorithm, which was also prepared as software named Actiware®. Different from the standard method, they used original activity count for the analysis. I suppose that many machines were required for their study, and the activity count from different Actiwatch® might be used for the analysis. I previously described the limitation for monitoring wrist movement by Actiwatch® [Kawada, 2012]. In this article, I quoted user's manual of Actiwatch®, in which the absolute value of activity count from different Actiwach® does not always agree with each other. Two-dimensional sensors are made precisely, and mounting and positioning affect raw data from Actiwach®.

I recommend conducting validation study for the direct use of activity count from Actiwatch® for the clinical evaluation. Although they tried to remove the placebo effect, reliability of activity count from the same machine and inter-machine variation are required to keep validation of their study.

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