In digital PCR, targets are amplified and detected using fluorescence-based probes. Positive (1) droplets, which contain at least one copy of the target, exhibit increased fluorescence over negative (0) droplets. In the ddPCR system, the QuantaSoft™ software determines the numbers of droplets that are positive and negative for each fluorophore (for example, FAM and HEX or VIC) in each sample. The fraction of positive droplets is then fitted to a Poisson distribution to determine the absolute starting copy number in units of copies/µl input sample.