Usually mixed up with ‘r’ but this is r-squared (also called coefficient of determination). This coefficient only takes values between zero and +1. R2 / r2 is used to assess the fit of the standard curve to the data points plotted. The closer the value to 1, the better the fit (yet another coefficient is the coefficient of variation (CV) which is the standard deviation divided by mean, and the smaller the value the less the spread of the data. In real-time PCR, CV is used to assess the accuracy of the results obtained in triplicate experiments). See also GraphPad Guide to Correlation Parameters and Interpretation of r.