Comparison of triplicate sets of relative peptide abundances in the absence of biological variability prior to and following normalization with the local regression technique. Scatter plots represent peptide ratios (ordinate) versus their mean abundances (abscissa) for (a) 111 common peptides from the standard proteins sample, block 3, (b) 1032 common peptides from the stationary phase growth sample of D. radiodurans, block 2, and (c) 1605 common peptides from the methamphetamine induced mouse striata tissue sample, block 3. Solid lines represent potential non-linearly dependent systematic bias estimated from application of the locally weighted regression and smoothing scatter plots (LOWESS) function.