Expression estimates (upper panel, see figure for assignment), their coefficient of variation and the ratio of the estimated and the experimental ("true") spiked concentration (lower panel) as a function of the spiked concentration. The latter two measures estimate the precision and the accuracy of the expression values, respectively. The expression estimates in the upper panel are scaled to agree with the diagonal (dashed) line which refers to perfect results. The perfect precision and accuracy refer to zero (no scattering, middle part) and unity (lower part), respectively. All values are averaged over all probe sets detecting spiked transcripts. The figure compares the performance of the hook expression estimates (PMonly, MMonly, PM-MM and R) with that of RMA (see text).