The invention is in the field of analytical technology and relates to an improved procedure for determining the concentration or activity of an analyte in a sample. Specifically the invention provides an automated algorithm for the quality control of (RT-)qPCR reactions. Plotting the fluorescence intensity of a reporter dye divided by the fluorescence intensity of a passive reference dye against the cycle number leads to a so-called sigmoid function which is characterized by a background phase, an exponential growth phase and a plateau phase. Since the fluorescence intensity as a function of cycles relates to the initial number of template molecules in the sample, qPCR curves can be used to quantify the amount of RNA or DNA fragments in the sample by determination of a so-called Cq value.