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Tremor is a rhythmic, involuntary, oscillatory movement of a body part (e.g., head, hand, etc.) The first objective tremor measurement was performed in 1889 and historically, measurement has been primarily one dimensional. Recent advances in inertial sensing technology have spawned interest in capturing motion information from multiple degrees of freedom (DOF). Our current interest is in the formation of a valid, single number composite score that answers the basic question: "How steady is the body segment under test?" Two approaches to steadiness composite score formation are investigated: 1) the traditional method that relies on addition, and 2) a new performance theory-based approach. Conceptual and experimental investigations were performed. The latter involved previously published 4 DOF data from 20 healthy subjects. It is shown that the multiplicative, performance-theory based composite score has better intrinsic validity and is more sensitive.