... The fitting package in STSDAS (Space Telescope Science Data Analysis System) relies on two basic techniques (one linear and one non-linear) for fitting functions to data. ... Results show that both methods may generate biased coefficients (at the ~few percent level), and over or under-estimate error bars (at the ~10 percent level), in particular from low signal-to-noise data. ... Function coefficient errors are computed directly from the covariance matrix. ... Abscissa is signal-to-noise. ...