Errors from repeated observations. The filled circles show the differences dz between the spectroscopic redshifts determined from two or more spectra of the same objects using the Hα line. If more than two spectra were collected, the difference to the average of the measurements is shown. The open circles show differences between redshifts based on the [O III] lines; the open squares show differences between redshifts measured using the Hα and the [O III] lines.