## 1 Introduction  
It is estimated that 1% of 70-year-olds suffer from Parkinson's disease (PD) [1]. At present, about four to six million people are PD patients, in which about 10% of PD patients are younger than 50 years old [2]. Due to demographic increase of the elderly population, PD will occur more frequently in the future.  
Parkinsonian tremor (oscillatory movement) is the central symptom of PD, presenting in about 70% of PD cases [3]. It can be judged in the patient's hands, feet, and head with a particular frequency (3.5-7.5 Hz) and amplitude (speed and range) [4]. Tremors often begin from one finger and expand to the whole arm, with the rhythmic back-and-forth action called the \"pill-rolling\" tremor [5]. The \"pill-rolling\" tremor takes its name from the action of rolling a pill with the thumb and index finger.  
Parkinsonian tremor manifests in different types: rest, postural, and action tremors. Rest tremor, which is the characteristic of parkinsonian tremor, happens when a body part is relaxed. The postural tremor occurs while a body part is held straight out from the body in a stable position against gravity. The action tremor (kinetic tremor) happens when a voluntary contraction of a muscle follows a certain action--for example, holding a cup [6]. Pure rest tremor is rare. For a PD patient, postural or action tremors appear together with rest tremor, but with different frequencies. A rest tremor may be combined in a postural tremor, but disappears during an action tremor task. An action tremor occurs in most PD; however, the tremor severity is not associated with age or disease duration [7].  
The precise mechanism of PD remains uncertain at present [8]. The Unified Parkinson's Disease Rating Scale (UPDRS), which is a subjective assessment performed by the qualitative judgment of neurologists, is the clinical standard for parkinsonian tremor assessment. UPDRS tremor ratings are from 0 to 4, which denote normal, slight, mild, moderate, and severe, respectively.  
Spiral drawing tests by a parkinsonian patient are used to judge the severity of parkinsonian tremor [7]. However, there is no practical instrumental method for the accurate monitoring of the PD symptoms as of yet. Nevertheless, doctors and patients are eager for objective severity assessment of parkinsonian tremor [9].  
In addition, about 50% of PD patients have side effects such as dyskinesia and motor fluctuations after five years of levodopa medication. The occurrence and amplitude of parkinsonian tremor are influenced by such effects [10,11].  
[PERSON] _et al._ reported that parkinsonian tremor exhibits a two-order nonlinear and no-strictly periodic oscillation [12]. Because the dominant frequency of parkinsonian tremor has no direct relationship with tremor severity, the tremor severity can be rated only by tremor range (_i.e._, amplitude) [13].  
To quantify neurological symptoms during deep-brain surgery, we have presented a wearable system for tremor quantitative assessment [14, 15, 16, 17].  
The goal of this study is to investigate the features of parkinsonian tremor and to develop a sensor-based quantitative assessment method. In this concept, the current possibilities of inertial sensor technology and motion-tracking algorithms can be used to implement quantitative assessments of parkinsonian tremor.