1. A computer-implemented method of determining a Ct value for a real-time PCR data curve having a parabolic shape, the method comprising the steps, implemented in a computer system, of: receiving a data set representing a PCR growth curve, the data set having a plurality of coordinate values (x,y); approximating the data set using a two segment piecewise linear function having a first linear segment ending at x* and a second linear segment beginning at x*; determining: a) whether an R 2 value of a quadratic fit to the data points beginning with x* to the end of the data set is above a predetermined threshold value of 0.90; and b) whether an R 2 value of the two segment piecewise fit over substantially all the data set is above a threshold value of 0.85; and c) whether the slope of the second segment is greater than the slope of the first segment if both slopes are greater than zero; and if so: estimating the Ct value of the data set by determining the intersection of the second linear segment with a mean value of a baseline subtracted horizontal line; and outputting the Ct value for display or further processing.