A circuit includes a sensor coupled to a processor. The sensor senses an electrical signal that is representative of a patient parameter, and the processor determines a condition of the patient by analyzing first and second overlapping portions of the sensed electrical signal. For example, a portable AED can include such a circuit to sense first and second overlapping sections of an ECG. By utilizing this overlapping-window technique, the AED can obtain and analyze multiple sections of ECG data, and thus can make a shock/no-shock decision, more quickly than an AED using contiguous-window analysis. Thus, the overlapping-window technique allows one to use both longer ECG sections (better accuracy per window) and more of these longer sections (better voting accuracy) over a given analysis time. Furthermore, this overlapping-window technique significantly reduces or eliminates boundary problems because the boundary of one ECG section is within the interior of either the preceding or the following overlapping ECG section.