Overview of experimental variations and control methods. (A) Exemplar sources of variations. Systematic variations affect all proteins or peptides similarly. Protein- or peptide-specific variations affect only particular proteins or peptides. Random variations are not listed because they are not controllable. (B) Control of variations by different quantification methods in the analysis of plasma samples on a depletion-MRM-MS platform. (C) Overview of the three assessment studies and the combined HPS dataset.