The penalized subset scan was evaluated using emergency department (ED) data from 97 Allegheny County zip codes. We compared detection power and spatial accuracy, with and without compactness constraints, on synthetic, spatially localized outbreaks injected into the ED data. Our results show that including compactness constraints allows the penalized subset scan methods to detect outbreaks earlier and improve spatial accuracy, as compared to the unpenalized fast localized scan and circular scan, over a wide range of neighborhood sizes (Fig.  1) . Without compactness constraints, the method averaged 7.6 days to detect and an overlap coefficient of 54.9% for neighborhood size of k 010, but detection performance degraded rapidly for smaller or larger values of k. Performance of the compactness-constrained methods was less dependent on neighborhood size, requiring 7.4 days to detect and achieving an overlap coefficient of 59.3% for well-chosen parameter values.