Abstract
The slope of the z-transformed receiver-operating characteristic (zROC) in recognition memory experiments is usually less than 1, which has long been interpreted to mean that the variance of the target distribution is greater than the variance of the lure distribution. The greater variance of the target distribution could arise because the different items on a list receive different increments in memory strength during study (the "encoding variability" hypothesis). In a test of that interpretation, Koen and Yonelinas (2010) attempted to further increase encoding variability to see whether it would further decrease the slope of the zROC. To do so, they presented items on a list for 2 different durations and then mixed the weak and strong targets together. After performing 3 tests on the mixed-strength data, Koen and Yonelinas concluded that encoding variability does not explain why the slope of the zROC is typically less than 1. However, we show that their tests have no bearing on the encoding variability account. Instead, they bear on the mixture-unequal-variance signal-detection (UVSD) model that corresponds to their experimental design. On the surface, the results reported by Koen and Yonelinas appear to be inconsistent with the predictions of the mixture-UVSD model (though they were taken to be inconsistent with the predictions of the encoding variability hypothesis). However, all 3 of the tests they performed contained errors. When those errors are corrected, the same 3 tests show that their data support, rather than contradict, the mixture-UVSD model (but they still have no bearing on the encoding variability hypothesis).