Behavior of the measured microturbulent velocity as a function of SNR, derived with the classical method (empty circles) and the prescription by Magain (1984) (filled circles) for a giant star. Each point is the average value obtained by using 200 synthetic spectra of a given SNR, and the errorbars indicate the dispersion around the mean. Dashed lines indicate the original value of v_t of the synthetic spectra. Each panel shows the results obtained by considering different thresholds for the residual errors in the EW measurement.