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A two-alternative forced choice (2-AFC) observer study has been performed to estimate the dose needed for detection of small cancers by digital stereomammography compared to projection mammography. Monoscopic and stereoscopic images simulating 4 different values of signal-to-noise ratio (SNR) were interleaved and repeated for 3 different object diameters. Five hundred images per condition were read by 4 observers and the fraction of correct answers and the corresponding d' as a function of condition were calculated. Preliminary results indicate that stereoscopy could be performed at 1.5+or-0.2 times the dose of monoscopic viewing. In our previous contrast-detail (C-d) study, we observed a factor of 1.1+or-0.1. The two experiments differ in target properties and decision tasks. In the C-d experiment, the viewer was asked to determine whether suprathreshold objects with an SNR of 5-6 were visualized in terms of a well defined edge and shape. In the 2-AFC experiment, the viewer was asked to detect an object at the limits of visibility (SNR range of 1 to 3). Experiments to elucidate the differences in observer performance are planned.