SD of each assay. The mean response for each sample (as per Figure 1) was plotted versus the SD of six replicates (intra-assay precision), eight assays on different days (inter-assay precision), or three operators on the same day (inter-operator precision). Circles represent donor 41; triangles, donor 68; and squares, donor 43. Open symbols represent CMV pp65495–503 responses; closed symbols represent CMV pp65 peptide mix responses (CFC and ELISPOT assays only). Error bars in the intra-assay graphs represent the SD of 10 times that the six replicates were repeated. Lines represent linear regression of the combined data (both antigens, where used), with 95% confidence intervals of the regression shown with dotted lines.