[Numpy-discussion] difference between ma.masked and ma.masked_array?

Hello, what is the difference between ma.masked and ma.masked_array? I am using this expression along with the scikit.timeseries: series[(series.years>2000)&(series.years<2010)]=np.ma.masked => but now, my series does not get masked. Rather the selected values (left part of the '=') get set to 0.0. But if I use arr_masked = \ ma.masked_array(series, series[(series.years>2000)&(series.years<2010)]) the masked is applied sucessfully. This is very strange. for some examples it works to use the firt method, for others not. Any hint is appreciated. Thanks in advance, Timmie