We present reverberation mapping results for the \hbox { { Mg } \kern 1.0 pt { \sc ii } } \lambda 2800 Å broad emission line in a sample of 193 quasars at 0.35 < z < 1.7 with photometric and spectroscopic monitoring observations from the Sloan Digital Sky Survey Reverberation Mapping project during 2014 - 2017 .
We find significant time lags between the Mg ii and continuum lightcurves for 57 quasars and define a “ gold sample ” of 24 quasars with the most reliable lag measurements .
We estimate false-positive rates for each lag that range from 1-24 % , with an average false-positive rate of 11 % for the full sample and 8 % for the gold sample .
There are an additional \sim 40 quasars with marginal Mg ii lag detections which may yield reliable lags after additional years of monitoring .
The Mg ii lags follow a radius – luminosity relation with a best-fit slope that is consistent with \alpha = 0.5 but with an intrinsic scatter of 0.36 dex that is significantly larger than found for the H \beta radius – luminosity relation .
For targets with SDSS-RM lag measurements of other emission lines , we find that our Mg ii lags are similar to the H \beta lags and \sim 2-3 times larger than the C iv lags .
This work significantly increases the number of Mg ii broad-line lags and provides additional reverberation-mapped black hole masses , filling the redshift gap at the peak of supermassive black hole growth between the H \beta and C iv emission lines in optical spectroscopy .