NOTE: None of the options FIXONE, FIXTWO, RANONE, RANTWO, FULLER, PARKS, DASILVA, GMM, ITGMM, BTWNT, BTWNG or POOLED   were specified. The RANTWO option will be assumed.
NOTE: A negative variance component estimate of -0.006382 was obtained for cross sections during RanTwo method
      computations for model statement . The estimate was set to zero.
NOTE: A negative variance component estimate of -0.0008511 was obtained for time series during RanTwo method
      computations for model statement . The estimate was set to zero.

什么意思，怎么解决？