Before revision:  Although the standard OPE and OPL assume the same distribution of covariate between the historical and evaluation data, there often exists a problem of a covariate shift , i.e., the distribution of the covariate of the historical data is different from that of the evaluation data.
After revision:  Although the standard OPE and OPL assume the same distribution of covariate between the historical and evaluation data, a covariate shift often exists , i.e., the distribution of the covariate of the historical data is different from that of the evaluation data.