We define a compound to be false positive if it was reported to be active in the primary screen but was found to be inactive or inconclusive in the confirmatory screen. Depending on the protocol employed for the confirmatory screen, different false positive types can be identified. When adding replicates, only errors associated to readout fluctuations or systematic errors (i.e. dust in the well plate) can be identified, while dose-response measurements enable detection of auto-fluorescence, colloidal aggregation, assay technology interference and so forth.