One fundamental problem in predicting the subjective quality of a degraded video is that the perceived quality depends on the properties of the video itself, or the context of the degradation. In this paper, we present the results for a series of ex- periments designed to measure the detection thresholds and annoyance values of small regions of MPEG-2 artifacts inserted into mostly uncorrupted video sequences. In previous work, we found that the detection threshold contains much, but not all, of the information needed to remove the dependence of quality on the context. In this paper, we report the result of two experiments. In one experiment, we varied the type of MPEG-2 artifacts inserted into the test sequences. In the other experiment, we varied the location, size, and duration of the corrupted regions in the test sequences. From each set of data, we estimated detection thresholds and fitted the parameters of a quality function. The experimental results demonstrated that, under a wide set of test conditions, the detection threshold is still very useful for the estimation of quality as context varies. In fact, the detection threshold was the only factor necessary to model the changes in the quality function parameters with artifact type. The experimental data showed that the detection threshold and the quality function parameters do depend on the size and duration of the degraded region. However, the effects of size and duration are minor relative to the effect of artifact location.