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Movement asymmetry is one of the motor symptoms associated with Parkinson's disease (PD). Therefore, being able to detect and measure movement symmetry is important for monitoring the patient's condition. The present paper introduces a novel symbol based symmetry index calculated from inertial sensor data. The method is explained, evaluated, and compared to six other symmetry measures. These measures were used to determine the symmetry of both upper and lower limbs during walking of 11 early-to-mid-stage PD patients and 15 control subjects. The patients included in the study showed minimal motor abnormalities according to the unified Parkinson's disease rating scale (UPDRS). The symmetry indices were used to classify subjects into two different groups corresponding to PD or control. The proposed method presented high sensitivity and specificity with an area under the receiver operating characteristic (ROC) curve of 0.872, 9% greater than the second best method. The proposed method also showed an excellent intraclass correlation coefficient (ICC) of 0.949, 55% greater than the second best method. Results suggest that the proposed symmetry index is appropriate for this particular group of patients.