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Several important clinical applications depend on accurate ultrasound image frame-to-frame motion estimation. Assuming that there is a degree of finite noise in the image frames and that speckle partially decorrelates between successive frames during freehand scanning, we hypothesize that an optimal inter-frame interval (step size) must exist that provides the smallest relative dimensional error over a set of accumulated motion estimates. Smaller frame increments suffer from less decorrelation-related inaccuracy but present greater potential for cumulative error because more estimates are used over any specific dimensional interval. We studied these effects using a combination of theoretical modeling, numerical simulation, and experiments. Components of diagonal motion due to the limitations of manual transducer movement were considered as the cause of decorrelation. The results were examined for four different angles of the diagonal motion and two different signal-to-noise ratio (SNR) values. These indicate that an optimal step size does exist and that this is dependent on many variables including SNR, angle of the diagonal motion, transducer geometry, lens focusing parameters, transducer operating frequency, and beamforming parameters. In practical experiments, we found that the optimal step size generally required using every available image frame rather than 'skipping' any intermediate frames.